Because you don't have to get quiet and sit down. Like Nilsson says, they want the crowd to get into it with chants and cheering and such. It's more like Davis Cup tennis in other countries, if you've seen that. I didn't know we host the SECT this year- I'll have to plan on going to that.
Like he said, you still have to be respectful of the opponent. That basically means don't taunt them for bad shots and don't do things like yelling every time they toss the ball on a serve. Other than that, there is play going on on multiple courts, so the players have to cope with crowd noise during points. So get into it.