"The Law is hard, but it is the Law." (7.99)
And Jace and the other Shadowhunters must adhere to it. No questions asked.
"My oath to the Covenant binds me," [Jace] said. "No such oath binds you." (8.60)
Here, Jace is telling Clary that she can tell Simon all about the Clave, etc., because she isn't forbidden to do so. We have a feeling that Jace wouldn't tell Simon anything even if he had a gun pointed at his head. There are some rules you just don't break.
"I thought the Clave was pledged to protect people." (10.83)
Sure, that pledge sounds a lot like a law, designed to get the Shadowhunters to do their duty. But things get grayer when you consider the fact that who falls into the category of "people" is debatable.