This statement typifies a common approach to D&D which I have always find ironic (and try to guard against with myself). No one balks at the idea that a Druid can change into an animal and can cast a spell without penalty because of a feat, but somehow an animal casting spells is so strange in the world of D&D that other creatures shouldn't be able to recognize the action as easily?
It's a mystery to me when/why people decide to impose a sense of plausibility or believability in a game in one instance and ignore it in another.
This isn't really about plausibility - both rulings are plausible, just different. The druid trains in it, so it seems fine to me that *he* avoids penalties. I just like to use circumstance bonuses/penalties in play to make (more) actions have consequences.