lets get the policy straight because you only relayed half of it: UAs policy is not to remove passengers who have boarded except when safety/security is involved.

Yes, there is obviously a question of how the pax boarded and made it through security without a ticket, and that’s a separate thing. But especially in this day and age, flying without a ticket is most definitely a security issue, and likely safety, too (wouldn’t have been counted in weight and balance, what happens if something happens to the plane enroute and the pax isn’t on manifest, etc.). This is not specific to UA - not too long ago, an ANA flight turned back to LAX after four hours in the air because of a pax onboard that wasn’t on that flight.

I’m actually not sure why people keep mentioning Dao in this thread - that was a completely different situation with different circumstances. I don’t mean to sound unsympathetic to the pax here - it’s a really crappy situation that certainly wasn’t their fault. But comparing to Dao is a bit much. And asking the tens of other pax onboard, many of whom likely had their own connections to make, while they sorted out ticketing for 1 which, to be fair may have taken a few minutes, or may have taken much, much longer, isn’t really the answer either. Deciding to hold a plane happens sometimes (and is decided by ops, by the way, not a GA), but I don’t know if I’ve heard of it happening for untickefed pax.