It’s not. Any business can put out a tip jar and any person can decide whether to dump their change in or no. There’s no societal agreement to do so. Feel free to keep your change - no one will stop you.
I wish we didn’t have to have this conversation over and over. Drivers get paid $8-10 an hour and usually get a very small stipend or nothing for gas, and use their own vehicle. If everyone gives them a $2 tip they make maybe $50 more per night. That makes a huge difference, that makes 20K a year 30K. It makes a person have a job with a livable wage. And all it costed you was a measly $2.