Tax stamps are an attempt to charge drug "offenders" with tax-evasion. Why is this important? Because being convicted of tax-evasion means they can confiscate various forms of property, be it your car, your home, or what-have-you. In many states these laws have even withstood constitutional challenges. It is a dangerous and irresponsible form of legislation designed to "get" lawbreakers by any means necessary.

Ignore them. Buying tax stamps is nothing more than self-implication and sets a dangerous precedent.

Edit: It's almost comical how the US Government fails to recognize that legitimate taxes on cannabis would prove very profitable. Yet it continues to endorse these tactics.