No one will reimburse you for the extra costs as it is the passengers responsibility to check for entry requirements. Have been the victim of the same thing last year when I had booked a Stockholm to Minsk flight via Moscow on Aeroflot. Apparently the Moscow-Minsk flight departs from a terminal that requires you enter Russia, thus you need a transfer visa. Even though I was frustrated to be denied boarding at the gate, I had only myself to blame. Since then I rigorously check the entry requirements for where I am going, even for the transfer points.

Unfortunate, but United assumes you take responsibility for knowing the travel requirements. In this case, UA would rightly assume you would have bought a return ticket from another carrier or travel agent. It’s not United’s responsibility to ask you about your travel arrangements. There’s actually quite a few countries that want to see exit plans on entry. At least you can tuck this experience away for future travels.

united doesn’t just ‘assume’ you take responsibility - they have already gotten you to agree you have taken responsibility for knowing travel requirements - that is in the COC. True, you don’t really ‘have’ to, as is the case here. But you already signed (so to speak) that right away when you purchase the ticket. Customs may have not asked about it, but they could have. And Copa didn’t want to take that risk. Not something I would be unsympathetic to.

maybe so, but it’s what customs officials appear to accept as proof. What else would be a better document with proof?

or there are certain things that are easier for a computer to verify then others.

Not having an appropriate visa is probably the number 1 reason for entry denial, and is easy for the airline to check. Return ticket is also easy to check, but given that not everyone buys a round trip ticket, not necessarily a matter of if the carrier bringing you in can see one. UA rarely checks travel docs at an outstation except at check in, usually only doing so at the gate of the departing international flight. If I check in online for a flight to say, Canada, using my existing passport that UA has in the the system, the outstation doesn’t ask me for it (unless I’m checking a bag). I only get asked, at say, ORD, when boarding the actual flight to Canada. If I left my passport at home, say, no one would know until I couldn’t board the plane at connection point.

Interestingly, other countries work differently. I’ve been on departing international flights on at least a half-dozen carriers out of India, probably more, and only UA, and maybe one other, checked for anything ID-wise at the gate. Even though it’s not legally required ex-US (IIRC), UA always checks when boarding international flights - this is for their own protection so they aren’t fined and forced to return a passenger back if they show up trying to enter a country without required docs.

Yes, as others have said, your responsibility to have an onward ticket. If need be a throwaway ticket on an LCC to a nearby country would have done.