I would draw a distinction booking a trip that you may be taking and the cancelling the next day, and rebooking for the same flight, you're really only tying up 1 seat that you're most likely using.
As opposed to booking a flight leaving Monday, another Tuesday, and another Wednesday, with the same O/D and then cancelling 2 of them as then you're tying up multiple seats with 2 of them you couldn't possibly take.
Or booking a flights all leaving Monday EWR/LAX, EWR/MIA, EWR/MSY again you're tying up at least 2 seats you can't possibly travel on.
The first scenario I don't think would be a huge deal (unless repeated an obnoxious amount of times), the other two I could see UA taking offense with impossible itineraries being booked.