If you're suggesting that lower-income people would be more likely to dispute the charge due to it having a larger impact on their bottom line, I strongly suspect you're wrong.
People in that demographic tend to, in my experience, not put forward the effort to get things like that refunded. Even though the money is important to them, it's frequently conditioned that it will take hours of time and effort to recoup the $5, and that's hours and effort they simply don't have.
Whether or not it actually takes that amount of time is irrelevant. The problem is that people expect it will take too much of their time to deal with and simply let the money go.
This is yet another example of Uber treating their customers like shit. If it's easy enough for them to determine that the cancellation was legitimate and issue the refund, they should do so proactively. Waiting for customers to complain before returning money they collected for a service they did not provide is as user-hostile as anything else they do.
I will never give another dollar of my money to Uber.