The documentation tells to create ie7.css and require the ie7 specific file from there. This only works if you have removed the default require_tree from application.css. If you leave that directive to application.css then ie7.css also gets bundled in.
Would be great if you could send a documentation pull request.
My example doesn't have a require_tree in it, so I don't feel there's an error here. Yes, we could improve it by being more explicit, but at some point, we're actually documenting Rails, not Formtastic. Closing for now, will accept a patch, but I'm not going to work on this.