I put unacceptable, based on if it wasn't disclosed.
I, personally would have no clue what to do if they got into my tank. I don't know the signs/symptoms, not the foggiest of what they even look like.
I put a lot of time and effort trying to educate myself on how to be successful at a marine tank. In my opinion, I am responsible for the lives of the critters in that tank. So I read, and waited, and researched more...
But there is some stuff you just won't find in the books for newbie aquarists. There are only so many photos, so many descriptions that can be crammed into a book. And without hearing about it, even internet search won't help.
I find myself Google-ing at least one topic, or abbreviation, or term used here, per day. There is just so much to know, that to do it in a shor time is impossible.
So, to sum it all up, I think it's pretty rotten if you don't let someone know you're giving them something that could harm what's in their tank.