Continuity mistake: In the first meeting with Casey Jones, Raphael jumps over Jones. As he does, Jones starts to turn around. As Raphael lands, Jones is starting to turn around again.
Continuity mistake: When Leonardo engages Shredder on the roof-top, there is a scene where Shredder is seen close front. In that instance he is actually fighting Raphael; on the right you can see his red mask, and the weapons Shredder parries are sai, not swords.
Answer: It's not really petty thievery. Shredder essentially has control over a large crime syndicate which is involved in pickpocketing, burglary, and highjacking. And that's just the gang itself. The Foot Clan itself is probably involved with other activities that we haven't seen.