Yeah, that's a trick drivers play on customers so they don't take a hit to their rating. If Uber catches a driver canceling too often, they'll boot them or freeze them from picking up more fares. (Conversely, if a customer is doing the same thing too often, a similar penalty of fees or the like will happen to them as well.)

So next time they try that, feel free to hold your ground by using Lyft instead or something. You don't have to cancel - they must, by their own rules, either cancel themselves, or pick you up.