Fanfiction: Why would you post on the internet something that you would not submit to your English teacher?

Here's my point. I don't read fanfiction, but I hear so much about it that I went and checked out a fanfiction website. As I was reading I noticed many grammatical and punctuation errors. If some of those were submitted as short story ignments in your English cl, you would get pretty poor grades. If you are going to post plagiarized stories on the internet, would it be too much to ask to have someone proof-read it first?