Those are both true, but they are also different. Your being given a Lexus doesn't injure anyone who paid for the Lexus. But taking a seat in E+ makes that section more crowded and devalues the experience of the paying customers.
Plus, as you point out, those are optional policies. The rental car company and the stadium are both within their rights to say no and give you exactly what you paid for.
And I see nothing wrong with United's analogy. They were throwing shade on a passenger who deserved it.