I can see why they'd want to ban browser plugins that affect their site. I've seen a few people create things like this which can cause problems for the users,

I can see why they'd want to ban browser plugins that affect their site. I've seen a few people create things like this which can cause problems for the users, and support. Many people install plugins which affect specific sites, or greasemonkey scripts, and then don't understand what is coming from the actual site and what comes from the plugin. Then, when the site is updated, the plugin/scripts can cause unexpected errors and the users often blame the site, not the third party code they're running. So, I think that actually plugins like this are irresponsible and I wouldn't want people creating them for anything I owned.