You concede that "decorum would be good," but then your logic doesn't follow.
If your family had scrimped and saved to afford dinner at the finest restaurant in town, would you then leave all your manners behind? Would you eat and behave like pigs, disturbing all the others in the restaurant?
There's no difference between the two scenarios. You're either respectful of others, or you aren't. Everyone else has sacrificed something to finish their education and obtain their degrees, why are you special?
Personally, I was very happy to have my family attend my graduation, and they remained silent as I walked across the stage. Which is how I preferred it myself.
No one is more special than another. I just can’t bring myself to get bent out of shape over this.