Anybody can take any information and hash it to add to the OTS tree. It's not something Amethyst is necessarily doing, though, Amethyst does support doing it.

OTS is going to add a merkle root. Adding additional hashes to the tree doesnt take up additional space on chain.

It doesnt matter how you feel about whether or not it should be in the chain. The stamp will be created and if you publish anything digital, that has potential to be added to the chain by anyone.