"I don't gank lanes that are behind."

Depends on the matchup. Sometimes its smart to stay away, especially with current jungle exp being so bad. Giving that top a double kill isn't going to help you win.

If the player can stay around his/her tower, then yeah, by all means try to fight for a second and back off to tower if it goes bad... but when people continually push and go for out of range CS when they are really behind to a Riven, it just doesn't make sense to help them.

Go to another lane and snowball them so you have a chance to win.