It's the law that must not be ordering some action. Laws generally provide e.g. regulatory agencies with some degree of discretion on how to apply a given law,

It's the law that must not be ordering some action. Laws generally provide e.g. regulatory agencies with some degree of discretion on how to apply a given law, like ban a widget. But that discretion can be defined by executive order. By contrast, if a law says an agency must do something, then an EO cannot override that law and direct them not to do that thing.