I would think people with basic income would be much less likely to commit crimes because a) essential needs can be met and b) they would have something to lose (freedom, time, money).
Beyond that, there's something inherently silly about your question - perhaps its the way it assumes the worst of people. You might as well ask why don't teachers get tenure and leave as much teaching as they can to subs without being fired, or why don't programmers pass all their work on to elance or odesk and get some additional outside work to make more money. These things may happen on occasion, but I'm inclined to believe that most are not so mercenary/sociopathic.
Beyond that, there's something inherently silly about your question - perhaps its the way it assumes the worst of people. You might as well ask why don't teachers get tenure and leave as much teaching as they can to subs without being fired, or why don't programmers pass all their work on to elance or odesk and get some additional outside work to make more money. These things may happen on occasion, but I'm inclined to believe that most are not so mercenary/sociopathic.