Because that's how Craig makes them.
*Runs around corner*
Lol. But seriously, I''m really curious about that. I mean, why have any of the mages' eyes glow at all?
I know someone's going to say, because it's cool.
But then why isn't it cool on a beastmaster or a priestess?
The world may never know.
I was going to say that obviously elves don't have glowy eyes... but the Johktari Beastmaster is human and also lacks glowy eyes, so that doesn't quite work. Perhaps she's secretly part elvish?
On that note, I did notice that humans and orcs tend to be portrayed as "evil" aligned (the Necromancer and both Warlocks and human and Dark school; the male Wizard is a pretty angry/unfriendly looking human; the Orc Warlord doesn't exactly give off friendly vibes, especially since he generally commands goblins and the orc butcher) while elves and dwarves seem to be "good" aligned (elves include Straywood Beastmaster, Druid, Priestess and Priest; and the Anvil Throne Warlord is like Papa Smurf of the otherwise friendly/good depictions of dwarvish soldier creatures in the game). The exceptions appear to be the Forcemaster and Johktari Beastmaster, who are both human but don't seem to be particularly "evil" - although I'm sure one could come up with fan fiction that makes them so. But the Forcemaster in the organized play story is definitely "good" in terms of being loyal to her friends and trying her best to keep peace between Westlock, Salenia and Sortilege while selflessly fighting against the "evil" Arraxian Crown and Bloodwave.
There are too many exceptions to your rule for it to be a rule. I think it would be nice if the depictions of the nations of Etheria were fleshed out a little better. I don't like the idea of nations being "good" or "evil" in the same way that individuals are considered to be that way. While even nations in real life can certainly be morally better or worse on average, sometimes significantly so, to label an entire nation "good" or "evil" neglects the more complex reality of incentives and culture created by where the nation is, what resources it has, its relationships to other nations, and what it has experienced in the past.
There is so much more potential for world building if you don't oversimplify the characterizations of entire nations. To start off with a couple of examples of how I like to imagine the nations of Etheria (note, these are not official canon):
The Arraxian Crown: They live on another plane of existence which is on fire all the time. Its denizens are burning and have been burning for as long as they've been there. They blame the people of Etheria for their eternal torture, and they want revenge. Their entire society is built on this. Their leaders are those who are strong and nasty enough to claw their way to power and then stay in power despite the constant pain of being burned alive, and who can dish out the most pain and fear to the Etherians.
Sortilege: A nation of scholarship and wondrous technology. They are one of the nations that was least scarred by the Demon Wars, and thus they have access to a lot of art, history, spells and technology that had been lost everywhere else. They are surrounded on all but one side by the ocean, so they are a good source of foreign trade for much of the Central Hearthlands. They are a bastion of science, culture and the arts. Pretty much all of their citizens are mages or training to become mages. Their current government started out as an experiment. For some complicated reasons that I won't go into here, it was hypothesized a long time ago that the stronger the mage, the wiser and more competent they would be in political leadership. This hypothesis was later falsified, but there has not since been enough political will to change the system derived from that hypothesis. A complex set of laws were passed to ensure that only those who had proven themselves to be competent leaders with good character could be allowed to duel an incumbent for their position, but so far these laws have been more difficult to enforce than anticipated.
I might write more of these fan-descriptions of the nations later.
I just think the eyes of all mages are glowing while they cast certain spells. Just a "photograph" of a certain moment. I imagine their eyes changing state multiple times during a battle.
Sidenote, my view of the mages ranked in order of evilness:
1) Priestess (goodie goodie for everyone)
2) Druid (goodie goodie but fierce protecting her forest)
3&4) Johktari/Straywood beastmasters (goodie goodie as long as they like you, but very fierce against invaders)
5) Forcemaster (Decent human being, trying to help own country, mind control is pretty dark though and "breaks eggs to make an omelet")
6) Anvil throne warlord (Decent person, quite selfish, Apart from rare situations only helps the clan)
7) Wizard (not "evil", but an asshole. Really selfish, only helps you when it can help himself)
8 ) Bloodwave warlord (Loves war and torture, but it's his nature/culture. Not evil by heart)
9) Priest (evil arsehole that thinks he is superior to everything else and views the rest as inferior/savages. Will annihilate everything with a different opinion. craves power)
10&11) Arraxian crown/Adramelech warlock (Burns with hate, love torture/destruction, craves power)
12) Necromancer (cold, emotionless evil. Wants power but doesn't have any emotion while killing which is even worse than the burning anger of the warlocks)
In my imagination, the priest is VERY far from good. only slightly better than the warlocks. Some kind of very dark/twisted crusader. There are enough awesome movies about those! 
Eh...I disagree. "Good" and "Evil" are not particularly useful terms for classifying mage classes. Better to determine their "moral alignment" by their training
Nature: values life and the natural environment
Mind: values intelligence, creativity, and self-awareness
Arcane: values knowledge, scholarship, and intellectual integrity
War: values combat, sacrifice, and wealth
Dark: values fear, uncertainty, and pain
Holy: values hope, faith and healing
