I guess because it's a bit like they're selling out, even though the BBC article a bunch of people have linked here says otherwise.
But really, imagine if Apple called OS X 10.10 "Pepsi" and you got a free upgrade with every Pepsi you bought. It just feels… I dunno, it feels kinda corporate and sketchy to me, and I think others as well.
I don't really have a problem with it though.