1) How would it not be legal? Do you actually think there is a law on the books that says one cannot be loaded onto an aircraft via beverage cart? Strange, but hardly illegal.

2) You don't think the passenger had some say before being place on the cart?

3) I think UA was probably reluctant to board the passenger in the first place. The passenger probably insisted (probably even waived liability) and they figured the only way to get him to his seat(s) was the method described. I further opine that he had 3 seats to himself and his leg to stretch out across. UA was probably taking pictures to cover their A.

4) I don't think you have enough information about the situation to pass judgement one way or the other. You were an observer and had no involvement with the decision making. If the passenger agreed to be loaded in that matter, what is it to you?

Edit - the Cessna Citation we used for our Russian operations to/from Japan was configured for a gurney to be loaded, if necessary. It required certain setbacks from the door of seats/bulkheads/etc. I doubt a gurney could make the turn required from the door into the aisle of a UA 738. A wheelchair apparently was not optimal with the leg condition, so the beverage cart was used. I hope he grabbed a Moscow Mule for the flight (and saved the bottle for later).