The answer is simple- because they are supposed to be allowed. By disallowing them you are breaking the agreed upon rules for the protocol. Before long it becomes impossible to implement new features because you can't be sure if someone else hasn't broken something intentionally.
I don't really have a dog in this fight about TCP 53. It does seem to me that it's a bit black and white to treat the RFCs as religious texts. It's important to follow them wherever possible, but frankly they don't foresee the bulk of the future security issues that usually materialize. So if a feature of the RFC isn't working for you security-wise, I believe it's your call to break with it there. As someone else said, don't complain when it breaks other things as well however.
If you don't like the rules- then change the damned protocol. Stop just doing whatever you want and then complaining when other people disagree with you.
I think its possible to disagree without calling other folks stupid... Best Regards, Jason