Isn't the key point here that the OP had a confirmed reservation on his IAH-DEN flight, and the CoC only allows UA to cancel this reservation if the OP does not appear at the gate by T-15. UA's action to remove the BP made it impossible for the OP to appear at the gate, thus it had the practical effect of cancelling his reservation. To me, this seems eligible for a DoT complaint. While I am unsure as to what compensation this may generate, at least it will begin to generate a DoT record that could force UA to correct the problem of surreptitiously deactivating BPs -- which will a benefit to us all.