[Cidan]

Federal legislation can not weaken state legislation in most situations where a state legislation is stricter. A good example of this is minimum wage at the federal and state level.

If legislation at the federal level were to apply a weaker standard for who can not be covered by a non-compete clause, a state can still expand coverage to all jobs.

[greglindahl]

Federal laws frequently explicitly preempt state laws. When CAN-SPAM was passed, it invalidated California's stricter spam law.

California has stricter auto emissions because Federal law allows it. Other states are allowed to follow the California standard, because Federal law allows that.

[ams6110]

Mostly federal laws prempt state laws by threat of witholding some sort of funding. When the federal interstate speed limit was 55mph, states could still have higher limits but they would lose their federal transportation funding.

[Cidan]

Yep; these are very specifically crafted situations though, which is why I said most and not all.