Because the ticket and thus the contract is from origin to final destination, not two separate tickets from origin to intermediate and another from intermediate to final. If UA were to board you at origin to intermediate w/out the requirements to complete the ticket/contract, and you were unable to meet the requirements to continue, the airline would be stuck on the wrong end of contractual liability. If you misconnect once you initiate travel, the airline's liability is to get you to your final ticketed destination. Now airline has accepted you without the ability to meet their obligation. As a side note, now let's say you have checked bags. Should they delay the hundreds of people on the intl leg to remove the cans, locate you bag, pull it, then reload the cans, or just let it go and eat the rather substantial fine and potentially endanger those onboard by violating security rules?

I'm no lawyer, just a CS agent, but how many regulated things allow 1 party to initiate a contract without having the ability to meet their end of it? Would you have planes take off into weather with minimum fuel hoping that the weather will clear and that they don't have to fly around/over the storm in your path or would you rather the airline take off knowing that without any unforseen circumstances, they have the ability to get to the destination?