Every polymorph spell that grants you special attacks in your battle form notes that you are trained with these attacks. They are also always unarmed attacks. Does your proficiency bonus for these attacks get a downgrade if you are usually an expert with unarmed attacks, and now you are only trained?
- "Trained" is not just proficiency in general, but the name of one of the proficiency levels in particular. The descriptions could have said "You apply your proficiency for unarmed attacks" or just nothing, because every class is at least trained with unarmed attacks.
- Animals are not usually very educated (see also: elephants with high strength and medium athletics)
- This could be meant to keep you from getting too powerful with these attacks or exploit monk abilities with them, as the attacks allow you to use your own attack modifier if it is higher than the (relatively high) modifier included in the spell.
- I seem to vaguely remember errata saying that yes, every class is trained in unarmed attacks. These descriptions could be a holdover from the time where that wasn't clear.
- There aren't many skill downgrades in the game, and those are always described as obvious drawbacks (literally, in the case of the silvertongue elixir.) It seems strange to limit your character's powers here and only here.
- If you did just want to say that players don't need to worry about proficiency penalties, should they want to use their own attack modifier, I guess that would be a concise way to say it.
Edit: The answers make me think I have asked this question in a misleading way, because I missed the obvious answer: You can be trained in a weapon and still be expert in its weapon category. I have added a suitable answer to explain.