Note: I got these ideas somewhere else. Though, they do make sense. First, legislators do make laws. That's inherent in the title. However, they are also judges, in a sense. They judge the President, and can have him removed with the right number of votes. Second, the Executive branch actually does interpret laws, to some extent - they do, of course, have to defer to the judicial branch. The president can even refuse to defend a law in the courts if he thinks it's unconstitutional. And we want the executive branch to play this role in interpreting the law. Third, the Executive can, with authorization under an act of Congress, issue regulations that are equivalent in enforceability to laws. That's why broadcasters follow FCC rules, and airlines follow FAA rules.