And if you are homeless, then you don't have a fixed address so you wouldn't be able to register to vote anyway. Are we going to say that voting laws are discriminatory towards the homeless now too?
Well, yes. I don't see any good reason not to allow homeless people the right to register to vote - you could take their biometric details even without a state ID card. Certainly you might argue that you would have less of a homelessness problem if an extra 3.5 million Americans to whom the issue mattered deeply were allowed to vote.