Whenever someone asks me for salt I make sure to not put any salt on.
Why would you do that?
I think it's the right thing to do
If someone asks you directly to do something you can easily do, it's rude to ignore them. When someone asks you to do something simple they are rightfully expecting you to accommodate them.
Why is this such a big deal?
You're just treating people bad by not doing what they've asked for. How would you like to ask for something and then be told you can't have it?