Why is altering the future limited to diary users?
The future only changes when a diary owner acts against the "reality" which their diary foretells. So the future for everyone is predefined, because they already decided if they want to eat the salad (but they don't know they did). Only if you know for sure that the current future contains you eating a salad are you able to change the future.

EDIT: 

About the Akise thing: No,  Akise didn't change the future. Yukiteru's diary never really tells the truth. It just tells what Yukiteru is/was/will be able to see. (It was the same thing when that little kid, Fifth, attacked them. Yukiteru was in the toilet and his diary didn't "say" anything except that it was quiet at the toilet.) So, Akise won the coin game, but he told Yukiteru that he lost, his diary will "think" he lost even though he didn't.