Web scanners do massive offensive attacks. They basically DOS attack your site in many ways, trying millions of attack vectors.
Mitigating against vandalism is very hard. It hurts users the more you do. Generally you leave it as open as possible and it is ok, since it's not a security issue per se and most sites can live their lives never having been attacked this way.
There's no money in vandalism and unless you piss off skilled or determined people it won't be abused.
Someone could write a script to cause thousands of $ damage to wikipedia without much trouble. But wiki chose's to leave itself open and take the risk. They don't have a bug. They are trying to do the right thing by users.
