Why does this happen?

That's an interesting thought. On some forums you see, there's a warning bar beneath the member's name. They get one warning (1st bit), then the second offence is a temp ban and another bit filled on the warning bar, then a 2nd warning (3rd bit), a longer temp ban (4th bit), and then a Perm Ban for their 5th offence (full bar). Then at least people do get a warning for doing something against the rules, and know that what they did was wrong. Because maybe when they did it they didn't know what they were doing was wrong.

Exactly, that was the case with me :P. Some people just have itchy trigger fingers ;_;