GoDaddy will not let you register a domain containing GoDaddy

If you go to Godaddy.com and try and register any domain that contains the phrase GoDaddy it will not let you complete the transaction. Try it. Register ljdasnxcweriGoDaddy84398sjdfkl.com. Won't happen. In fact, it will not even give you suggestions.

Register.com has not problem with domains with Register in it.

What's up with that?