Because when I do that at my sister in laws home I have to listen to a lecture about how consuming too much salt is bad for your health. In my own home I noted that even if I feel that I could use a pinch of salt in something other guests do not ask for it but when I do put it , they will too.
And that's why you clarify in advance like I said.
I feel that when people invite you they have obviously put in the effort to prepare a nice meal, at least to the best of their capability. So it's a bit rude to show any kind of negative.
We have a friend who doesn't really cook well, at least to our taste, but at her home, we usually pick one of the 3-4 items that we like the most and focus on appreciating that.
Hey, at the end of the day, if you make someone happy, or at least not hurt their feelings, that's good enough.