The funny thing is, "the customer is always right" is an expression that many companies adhered to (and many still do, but IMO less than before) for quite some time, and while there are some obvious flaws in that philosophy, there are a shitload more flaws in the idea that the customer is never right.
You pay for a service, you expect it to be delivered. If it hasn't been delivered, you demand it. It really is as simple as that.
Furthermore, if this guy hadn't acted like this, then United would still feel that their method of operation was correct.
It's best if all parties did the right thing.
In this case, United made the mistake of not accounting for their internal seat requirements, and instead of paying (extra) for that mistake took it out on passengers. Fortunately we have a fair enough court system which'll hopefully make things right.