I completely agree with you. But there may be some kind of security issues with IE. Some vulnerability to site's authentication system or something? not impossible, you see.
Not strictly true, most sites rely on the client's cross-domain policy for security. Without it, this site would be able to read your Gmail & facebook pages. If a useragent came out that had a major x-domain hole, I'd expect data-sensitive sites to block it.
Not that IE fails in this way (and blocking IE in this case it wrong).
+1. Valid point but this is a bit different to what I was referring to and is an insecurity in the browser rather than the application. Still, fair point. I was mainly referring to people validating input only on the client-side, redirecting people away from private parts of the site using JS or meta redirect (last year I had to maintain a site that used this), etc.