This is not only a problem in the reserved areas. Disney tends to NOT specify whether an area is standing or sitting. Instead they leave it up to the crowd. If everyone's sitting, then it's a sitting area. If everyone feels like standing up, then that's what you're expected to do.
I actually have the exact opposite problem as you - I enjoy standing for fireworks shows. And when I'm in an area where people are already sitting down, I'm expected to sit down too, which I think is somewhat unfair. My feeling is people should be able to stand up for the length of a fireworks show. But I've been overruled more than once by yelling people, so I just sorta skulk off to the side where others are standing.