The company does not, per se, have the means to verify all the weights on every pkg. I had asked before why a notation in the board couldn't be programed alerting us that a package at a particular stop was an over-70. That was the answer I was given. Rather, they'll look for discrepancies and red flags, and then confront you on them.
As for why drivers do it, it isn't always about the money. Ironically, it is often used to boost up time on the over/under report to soften the blow of an ugly over-allowed day. While technically one cannot get fired for performance, consistently being on the report may put one on the radar of the higher ups, and they could begin scrutinizing your methods and work day, and may find some legitmate way to discipline you, probably along the lines of failure to follow methods. It can be a fine line one must walk; however, it will take time for someone who is grossly overallowed to reach the level of severe discipline, whereas inflating over-70's is dishonest, a cardinal infraction that could lead to immediate termimation with no recourse. Take the lesser of the two evils.