Because most people know that they shouldn't use the gun.

When I played CS and CS:CZ, the auto was never banned in the servers that I frequented, it's just that people would never use them. There was a sense of understanding that the Auto was not a weapon you don't use. This carried over to Source for a while, especially in Beta and such, but ever since I came to War-Lords, auto use as run rampant.

If you've played CS for a while, and during the hey-days, you'd understand where those players are coming from. You just don't use the gun.