Also keep in mind, only staff members and the user can actually see them. No one else can see the mark, it's all kept private.
Also, the reason why it stays forever is so that if someone gets marked for trolling, and then does it again in a few weeks or few months, the staff can look back and say "yeah, he was marked for trolling once already", and then can make a more informed decision on the next time.
"You lack the requisite spine and testicular fortitude to study under me"
"You do not know the first note of the music that moves me."