I can almost guarantee that if they don’t already, the robo-taxis will eventually start asking for tips.
This is already the case at self-checkout in some stores for example.
As long as the companies can get away with it, they will tack on any number of extra fees and charges even if those fees and charges really don’t make any sense.
Hell, even tipping people does not really make sense the way it works in some places. A person working for a company should receive enough pay from the company itself that they don’t have to actually rely on tips in order to make enough money to survive. Tips should be a nice extra that customers willingly add because of good service. Not a forced extra percentage that they have to pay on every transaction just so that the company can pay less to their employees.