Woah - a lot of animosity here for something that a bit of research clearly shows isn't allowed! California Grill hosts a special New Years Eve "experience" for a whopping $675 a person - only those attending that event are allowed. California Grill closes to regular diners at 10 for this reason. Yes, it would have been nice for them to mention this to you sooner when it became apparent that you were stretching out your dining time to see the fireworks.
If you're skeptical, check out the NYE info on California Grill's page straight from Disney's website!
https://disneyworld.disney.go.com/dining/contemporary-resort/california-grill/
(Though it looks like you weren't there tonight, but another day they did the special fireworks? I would hope that wouldn't be the full $675 a person, but it still seems extremely probable it was for a special event. Either way, I can see why they'd have an issue with people staying two hours past closing time.)