I do the same thing when flying, if a plane is on a 10 mile final and I can jump in and turn final and exit the runway in front of him and don't cause him to have to go around, do a 360, etc. I don't think I have violated the rules.
I agree. 91.113(b) says that when another aircraft has the right-of-way, you may not pass over, under, or ahead of it unless well clear. Unless the aircraft on a ten mile final is a really fast airplane, I would say that turning a one mile or half mile final ahead of it easily qualifies as "well clear."