Am not sure why because I have never been banned from Facebook. You can check the rules for Facebook or search on Google why you have been banned and try to sort it out. Maybe you did something that defies there terms and regulations.
Whenever I would come across a nasty post what I usually do is to report the post to Facebook. The poster would be suspended for a week or so but when there are several reports of the misdemeanor then maybe Facebook would ban the member. There are community rules and regulations that Facebook is implementing so we should be aware that you cannot post just anything on Facebook. There are restrictions to be observed.