People return their shopping carts because its the right thing to do, but also there is minimal cost. As the cost ramps up the calculation for a 'selfless' act is effectively inverted to bias self interest instead.
This is how you get the current state of the financial sector. Ironically altruism is so cynically dismissed that its actually used as a marketing strategy to pursue self interest, with no one bothering to disassemble it because no one believes thats the motivation anyway (see: subsidizing homeowners with bad credit using homeowners with good credit). Leaving the only people who do believe it functioning out of partisanship and lack of curiosity.
'Abnormal' behavior is exponential in its effects. Much like a fire, it only stops if contained. Thats why total laissez faire deregulation is naïve. Even if most people do the right thing, society is not silo'd from the minority who does the bad things.