I don't see that the sites assume a person has effectively unlimited time to build up karma. What I see is that they aren't interested in the participation of a person who doesn't.
And "unlimited" is an exaggeration, isn't it? Isn't the actual amount of time quite bounded and even predictable?
Computer networks used to be full of communities like that. I used to read and contribute to several forums every day. I knew most of the participants and we discussed things I was interested in. I had to stop when the Net got too big and all of them turned into help desks. I can understand people trying to set up such a community again, at the expense of the occasional quality contributor who just drops in.