Supreme Court has decided that the final arbiter on rule making between statutory lines is on the courts now though. IRS can promulgate a regulation like that but anyone can take it to court and get their case ruled on separately.
Courts have always had oversight to some degree. Even when new statutes are written, you don't know if they can be enforced effectively until someone challenges the statute to create precedental case law validating it doesn't violate rights or anything.
