Thank you for reaching out to me. I'll get straight to it...No you are not unreasonably upset. I would be. And, the fact that one team got the warning to adjust their hours and the others did not, seems quite unfair. Usually, a "work social" is scheduled during work hours, and PTO isn't even in the equation, especially if it's a potluck, of little expense to the company and only 30 minutes.
It irritates the hell out of me when a company won't give something to recognize the team's contribution, build goodwill between departments, and have a little fun in the process. It seems as though the boss either was instructed to do this, and didn't push back on her team's behalf, or she is out of line asking you to pay for a social and bring your own food. It may be helpful to give your boss some feedback if you’re given an opportunity.
If you don't agree with the "rules" of this little soiree, then, by all means, don't attend. Instead, start your holiday celebration after work elsewhere! Sometimes you have to stand up to the boss, just be professional about it. I hope this helps!