SO IT WAS YOU!
LOL, just kidding.
If it was against a rule I would be more than annoyed and would probably say something to them.
For that matter I do pay for a spot. But I also know what it is like to come to an area with extra space, grass, etc and see a car parked there with no one tailgating.
Its an annoyance, not a huge deal but annoying. Simply, if you are going to utilize the spot, great, if not why not park across from that spot where there is less space to tailgate or no space to, And leave that spot for someone that wants to utilize the extra space.
I guess It seems like an unwritten rule to me, and one of basic courtesy. But then I guess I have seen a lot of uncourteous people at times in the lots, rules on not.