Sure, but it seems pretty simple. I don't think there's anyone who can be declared well and truly 'brain dead', so ending them prematurely just seems wrong.
Now, if there was a situation where you're causing more suffering for the individual by keeping them alive, then that's transcended from preserving hope to torture. In a case like that, then yeah I could see the moral compunction for ending their suffering. Not sure that's particularly common though.