1) Registering to vote isn't that hard. If you can't be bothered with completing a simple form, how much time are you going to spend learning about candidates or issues? Voting and registering should be as easy and simple as possible, but you should have to take some sort of proactive action in order to accomplish either task.
2) As voter registration databases are public record in most states, automatic voter registration essentially creates an index of every citizen, which raises privacy concerns. If you want to be off the grid (and not vote), you should be able to.