Enforcement on this varies greatly.
Vienna's TMs are rabid about it. But they also get verklempt about gatorade, lol.
Most places take a more common sense approach. The concern is you don't poke holes in the turf. As long as your chair has the little plastic things at the bottom, I wouldn't worry about it. If the team has a portable bench or canopy and that's allowed, no one's going to get upset about you sitting in a chair.