Interesting that the idea of ownership makes one ok and the other not ok. Destroying an irreplaceable object, especially one of some cultural significance, is still wrong in my opinion.
After all, it's not like anyone was unaware that the Chinese government has been destroying the country's cultural history wholesale since the 1940s.
But then, I get upset when musicians break instruments on stage at the end of a set. It shows a lack of respect for the makers of the object that I dislike.
At least a piece by Ai Wei Wei is replaceable, somewhat. Unlike the piece Ai dropped as a statement. Destroying the irreplaceable for political commentary is no more commendable than destroying current work is.
I have no problem with deeming certain items unacceptable, and enforcing that stance.
I do have problems with deceptively destroying someone's project. If the piece is unacceptable, tell the student upfront and reclaim the clay. Publicly reinforcing the rules is a far more productive action to take. Treating students with the respect you would expect is the least you can do. And kids are under no obligation to satisfy your paranoia. The more you push them, the more likely they are to become ever more evasive.
(When I was at culinary school, there was one chef who hated parsley. If it showed up on a plate, you were in trouble!)
Also, as noted above, if a kid who wants a bong can't make one in ceramics class, she will use a coke can or a plastic bottle. You may be upholding school policy, or your own opinion on the matter of smoking, but you sure aren't stopping kids from getting high or smoking anything else.
Deliberately destroying a piece without telling the person who made it why it was destroyed is just being a ########. Especially through deception. If the school you work at won't let students make certain things, tell them so, then tell them what the piece they made isn't acceptable.
The scenario as described is conniving, underhanded crap.
Also, how much is it your duty to police what your students do with their pieces after they leave your studio? You could hardly be found complicit in any criminal action, which I could see as the only real reason to care. Smoking is not yet illegal in the US, so making ashtrays shouldn't be any problem. No one is going to make you smoke by building an ashtray in your studio, and you aren't going to get them to quit.
(I remember being in grade school, and one of the most promoted objects to make was an ashtray for mom and dad.)