If they take a detour maybe they'll end up with a demerit for deviating from the optimal driving route and be docked pay - if they come back to distribution with a bottle of pee the company may also dock them for something like... I dunno "storing human waste in a vehicle used for transporting food stuffs" then dock their pay or fire them. Amazon is strongly motivated to discourage drivers from coming back to their shipping centers with bottles full of pee since either labour organizers or reporters might jump on that to use against the corporation.

All this could just be communicated through pretty innocuous sounding employee guidelines (maybe, "there can be no open bottles of liquid of any kind when a vehicle is in motion") or just word of mouth. A manager mentions to a senior driver that there will be some bonuses if nobody gets caught with a bottle of pee and that causes the initiative to disseminate through all the drivers.

There may be far fewer choices here than it appears and the best choice is probably to get OSHA involved and maybe get some new worker safety and rights legislation through congress.