In Fahrenheit 451, why would a society make being a pedestrian a crime?
"Society" in Fahrenheit 451 controls the people through media, overpopulation, and censorship. The individual is not accepted, and the intellectual is considered an outlaw. Television has replaced the common perception of family. The fireman is now a burner of books rather than a protector against fire. Books are considered evil because they make people think and question — and they might question their current circumstances! The people live in a world with no reminders of history or appreciation of the past; the population receives its whole worldview from television.
In Bradbury's dystopian world, the people in charge want everyone else to stay in their homes and watch TV. Porches no longer exist because they don't want people outside, where they could look around their neighborhoods and engage the people they live near.
Clarisse tells Montag that her uncle was once arrested for being a pedestrian. Why would this be illegal? If a person happened to look outside and see someone walking around, he or she may be encouraged to do the same. Walkers get the chance to explore their environments and meet each other. This would lead people to start to question things . . . which would then lead to discovery and the development of critical thinking (and intellect!) — exactly the things that the powers-that-be want to avoid.