why would society make being a pedestrian a crime?

In Fahrenheit 451, the government implements all kinds of rules and regulations to keep people inside. It is far easier to control the population if they watch television instead of meeting with friends, if they're in their homes instead of outside living. Individualism is non-existent, and those few individuals who slip through the system might actually give others ideas. Thus, something as simple as taking a walk might give others ideas. It was for this same reason porches were banned long before. People gather on porches...... they talk and share ideas.