The purpose of a constituion, any constituion, is to set up the framework for government or other organization. It is a necessary document, at least in modern times, along with "bylaws" for a functional organiztion. I don't think it implies restrictions so much as it simply says, "it works like this." The amendments, some of them, do set up restrictions on the power of the government, as well as other things.
Remember also that we are supposed to have a government of the people and by the people. It works a little better if you don't view the system as us and them.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.