It is not "definitely illegal" to set license terms to whatever one wishes including restricting commercial use. The GPL hinges upon copyright to enforce it's provisions. Copying privileges are revoked for people who violate the license. This is a legal "hack" to get around the need to establish a contract or exchange funds. The same sort of rules can be applied for any other criteria such as the type of user. If I want to prohibit blue-eyed people from using my software I am free to do so. Dual licensing isn't a requirement to do this.
