When money changes hands everything changes. Expectations both from the customer and in terms of legal liability are so much higher that you cannot compare gift or free exchanges to a fundamentally commercial one such as Uber or AirBnB. There is all sorts of really interesting research into this from the psychology side showing that things shift the instant people see something as a monetary transaction instead of a social one.
The services like to act as though they are some hybrid between the two (Lyft is particularly over-the-top about this) but they are not. Just as you can't be "a little pregnant" you can't be a little commercial in nature.