If people want to stand for an entire game, go sit in a student section at a college basketball game or sit in the last row of the balcony.
This is a professional basketball game. You're gonna be surrounded by people from all walks of life, who believe it or not, are Celtics fans also. They paid good money to go and cheer on the Celtics in whatever they want. If they're not as vocal or rowdy, so what? Doesn't mean they're not a real fan. They shouldn't have to spend half the game yelling at someone to sit down because the person can't figure out the opportunities to sit & stand.
Yes, there are instances when its fine to stand up and go nuts. And about 98% of the people there realize those situations; after a big run, a big dunk, huge block, huge 3, etc... Standing up just for the heck of it when Mo Williams is bringing up the ball and nothing else is going on just isn't one of those instances.
People show their passion in different ways. Just because someone isn't screaming their lungs off and standing up when you want them to stand up, doesn't make them a "real fan".