Regional Subtleties

[Cam]

But in general, the Federal Government can't just arbitrarily interefere with or nullify state law.

The federal Government in the US has been known to interfere with state law. Probably the most well known example is state drinking laws. The Federal government threatened to withhold state highway funding if states refused to instate a drinking age of 21. This might not be arbitrary but it does give the Feds the ability to change things to benefit the country as a whole.

Any state laws that people feel are unconstitutional can be brought to the Supreme Court. This may not be the easiest thing to do but it is a possibility that prevents the states from passing unjust laws, which in return brings about decent uniformity throughout the system. The US court system is a gargantuan mess because of this. However, it works and the majority of US citizens would not want it changed, and due to this being, a Democracy it will not.