It's ironic that Amir's cricket career started out with friendly bets, when he challenged renowned tennis-ball batsmen from Lahore and Rawalpindi to hit him for a six in one over and win a meal at any big restaurant in Islamabad. He bowled six dot balls.
"[The batsman's] shoelaces came undone when I bowled him a yorker," Amir said.
It's reasonable to assume that Amir took the same attitude in International cricket: What's the harm in making a little bet (and getting cash) by bowling a certain way that most likely won't affect the final outcome of a match?
Of course, that was utterly stupid. He got caught and paid a terrible price: he lost three years of precious cricket and his reputation is dirt. It will take him years to earn respect and any off-days in the field will invite rumors of possible, probable cheating.