In my experience, sharing of desserts (or appetisers, to hark back to the parent thread) is negotiated in advance.
It's not uncommon for me to be seduced by the description of a dessert, while perfectly aware I don't really have room to eat it. I'll ask, before ordering, if anyone would like to share - and if everybody else wants their own, it's up to me to order or not order. And between the four of us (DH, DD, DS and me) we'll often swap spoonfuls so that everyone gets the chance to taste everything. If we want to share a dessert, we'll ask for an extra fork or spoon.
I'd be unimpressed with a waiter who cheerfully assumed that my creme brulee, or cheesecake, or chocolate cake with wasabi, was generally up for grabs. It's not his call. And there is a distinct air of "you ought not to be eating All This Dessert" in an unrequested distribution of forks, which I would certainly resent if it ever happened to me. So my conclusion is that a waiter is in the wrong if he assumes sharing and provides extra cutlery when he has not been asked for it.
I don't think I'd have a problem cheerfully announcing that This Cake Is Mine, though.