Why does Uber allow drivers to scam customers with a bogus cleaning charge fee?

No one is stopping you from reporting the driver on the Uber app.

I'd imagine this is specific issue with this one uber driver; like the other person stated report it to Uber.

The reason for these sorts of charges is the poor Uber drivers would get stuck with having to pay and clean their cars for drunks that puke in their back seats. I would simply report this driver and let them know you did not have a drink in the car.