Hmm, I confess that I have done that my entire life, and until maybe a month ago it literally never crossed my mind that it would be a problem. I mean, sure the person is about to pay money to go see/experience/inject whatever noun they have just invited you to join in seeing/experiencing/injecting, but their willingness to do so does not change the basic fact that this noun still offends the universe by the mere fact of its existence. I guess maybe I hate it when other people do it to me? I think I only hate it when people with stronger opinions than me do it, which might explain why I don't think it's such an issue. I think this makes me an asshole?
 

What made me feel ashamed on this particular occasion was that I was pretty sure that the invitation was for me to attend at his expense. It's one thing when someone says, "hey, do you want to go see Outfoxed at the Berkeley Student Union'" when the clear implication is that you'll pay your own way. It's another when a co-worker offers you a free ticket, and then to say what I said. And also, there is also a difference in kind when a buddy offers you a free ticket versus a respected co-worker who you're friendly with, but not buddies with.

So, to put it in a personal context, when I asked if you wanted to go see Wal-Mart: The High Cost Of Low Prices that time, and you swore at me for 10 solid minutes, I didn't take that as rude, because I was going to make you pay for the tickets for both of us anyway, since you are a rapacious free-market incubus of Evil. And also we're buddies (if I may be presumptuous).