bad attitude
You could have whipped out your 1k card and said, "Is this how you treat your 1k members?"
I was on a flight and this 1k dude was next to me in business class. They didn't have his food selection, and he said, "What? 1k isn't good enough any more?" He got his food selection no problem.
But there is also a point N965VJ made. If you show up in a blazer and a dress shirt, they tend to give you more respect. If you show up with a backpack, sweatshirt and jeans, people may not. I saw on a travel show where the guy said, "I always travel with a blazer." It definitely makes you look more credible, even if you're off to hawaii on vacation. However, if you're a woman, then it may be different.
But no matter how you were dressed or what milage plus elite leve you were at, that is absolutely unacceptable behavior. I would complain to United until I was blue in the face if I were you.