I think it's fair to leave on time.
Why do you think it's fair?
I think it's a considerate thing to do.
It's normal to expect others to be on time to things, otherwise it will be difficult to plan our days. When someone is late, it can put other people behind schedule as well and it can also be annoying. Try to be on time whenever possible, and be sure to apologize if you are running late.
It's not a big deal if I'm not on time and I wanted to leave early anyway.
If you aren't on time, it can ruin things for other people. You should really try to be on time.