What I'm missing here is that Airbus is one of the parties who doesn't want to see US fail.
No one is mentioning the fact that US is operating a rather large fleet of the A320-family, more than 100 planes. They also have some 19 A320-family planes still on order as well as 10 A330's.
If US went down the drain, a large amount of A320's will be coming into the market, which makes new sales rather difficult because the owners of these planes will offer them with lower prices because of the amount of planes being returned.
Helping US out with a loan of $250 million is just keeping your whole business safe, not only the business of US. In the end, US didn't even needed the money. Don't forget that the loan also resembles trust in the airline, which can reflect on other companies who were maybe doubting if they would help the airline out or not.
Ordering the A350 is maybe a "thank you" to Airbus for helping them out and ordering Airbus keeps their fleet-structure simple as the plane is complimentary to their fleet of A330's.
These deals are very common in business land, be it in aviation, cars, trucks and even home furnishing.