UA already has the money in its pocket, why ask for more for the privilege of getting the seat to sell a second time? It makes no sense and I personally won't have a problem checking-in for a flight I don't intend to take if it were me.
Agree that it's silly to pay UA for such a privilege. It's also silly to check in for a flight you don't intend on taking. UA's OPS struggle enough as it is, the last they need is another wrinkle which further down the road may inconvenience fellow travellers. Whether OP checks in or not, UA is still getting money to sell the seat a second time, the only thing the check in does is delay selling the seat again to the gate. At some point (unsure when) UA will conclude that while OP did check in, they're a no-show and will clear their waitlist, thereby selling the seat yet again.
-James