You described exactly the problem people have with this idea :)
> Do you think that Tesla should have to make sure that its self-driving software is compatible with every ridesharing app that any random person comes up with? How will that work with liability if somebody sends their self-driving Tesla out to work under a ridesharing app that feeds bad inputs to it?
Then they should prohibit interfacing the car with 3rd party software quoting relevant regulations (or at least a generic "for safety reasons"), not put a blanket ban on commercial use of the self-driving functionality.
For now it seems that they don't even want me to physically sit there in the driver's seat and do something else while the car is driving, which puts your whole speculation about Tesla's motives into question.
And BTW, what if somebody breaks this rule and hacks his car into an autonomous Uber slave? By your logic, if something bad happens Tesla will still get blamed. All they accomplished is CYA and for that a warning about dangers of interfacing with unauthorized software would be sufficient. Even better in fact, because not all unauthorized software must be commercial.