If map makers don't want people on roofs, they'd put invisible walls on them.
Like i said, exploiting something that the map makers never intended to have happen.
Lets take for instance the valley map full of trees and bushes, you can strafe up the walls and get on top. You dont even need a ladder for that. Do you think that was intended? Nope, people saw you could do it, and now they do it. Anything to get an advantage.
How about the hill map where one team crosses a bridge and most of the fighting happens at the ruins of a wall, Every round someone gets on top of the tower and is the last person to die via suicide. I'm sure the map maker was thinking, Yeah someone will want to get up here and wait 5 minutes before suiciding, cause thats fun.
The same thing with ladders. A lot of maps have ladders in place that you can raise and lower. Clearly these are the roof's the creator wants and intended you to get on. But yeah lets make it HARDER on the map makers, and not ingenuity of people who like to exploit stuff.