I actually got caught on one of these from Adobe - I was doing a flash install, that I'd done many times before, and so was on auto-pilot. It's a bit like clicking send on an email and then remembering you needed to add an attachment (thank you Mozilla for attachment reminders!); a split second after clicking I realised that what I'd clicked on was not the normal license or whatever - by that point it's already installing.
The only objective measure is "is this software required for what I'm installing"; clearly, in this case, MacKeeper fails. There is a degree of trickery, sure, but they are unequivocally trying to trick people into installing something other than what they intended.