Hi all, first-time poster here. I'm one of the folks on BGG who was arguing about this (uploaded the same avatar for your image-matching convenience). I wasn't the one who started it, or even the first person to question the rule or suggested something different than what folks here are saying, but I was perhaps the most vocal at the end.

The thread is there for anyone to read, but for those looking for a summary of the other view, it's mainly that:
1) Having actions which are not verifiably legal at time of play is awkward at best, and generally not a desired design trait.
2) The game is better for allowing bluffs. It costs the player mana and the opportunity cost of both planning/playing something else instead at the time the enchantment is played, and of using the card at some point in the future.

If you don't want players doing that, I would rather associate a penalty for revealing a spell with an invalid target than say you can't do it. Then players could weigh the risk of getting caught with the value of the bluff.