Personally i don't see anything wrong with using tissue to make sure you have a seat. I do find those ppl who fiddle with other's stuffs rather rude.
I meant those ppl had worked in such areas whereby it's a norm to do so, why would u go and fiddle with people's tissue holder. when that happened to me, I just walked up to them and said, that thing is mine, and because they had fiddled with it, I decide to throw it away, which i did in front of them.