Actually some people would disagree about the prison start allowing you to create your background. Like if people want to play a completely "pure" or "good" character, they wouldn't want them to have gone to prison already.
Some "good" actions are legally wrong. Stealing medicine to save somebody's life, is just an example. You could have been framed (as mentioned), you could have been involved with a group you didn't know was doing illegal things until it was too late, you could have some ability (or disability, I should say) that makes you sometimes randomly cast a spell, and you just happened to hit someone this time. The possibilities are really endless.
However, I'd like it if I could start with a clean slate, like exiting a carriage into a city, or somebody pulling my character up onto a boat after they found him/her floating in the water.