One of my kids is a badge wearing, gun totin' Federal Enforcement Officer (ICE) and we have often discussed the difference between Federal law and the agents' ability to enforce that law. Since officers are thinly spread, they tend to enforce those laws that the government currently deems important and and only cursorily enforce those that it doesn't. The situation at the California border is a case in point. If you entered CA during the 'seventies, on I 8 or I 10, you would have been stopped by the CA state aggie people, asked if you were bringing live plants of critters into the state and then, unless you raised some kind of red flag, allowed to proceed without inspection, otherwise the roads would have been backed up across AZ. Now, those nice, sheltered checkpoints, (try handling a dog when it's 115F outside!) have been largely taken over by the BP feds, because catching EIs is considered much more important and newsworthy than catching Mexican fruit flies (much like the American ones, but with little sombreros).
In your case, Butterfly, since you were going into Colorado (I 15?) you shouldn't have been stopped at all (would you let us know what actually happened?).
Nor is it true that federal law automatically overrules state law in any given state. The rule of thumb is that which ever set of laws is the more restrictive applies. In CA, for example, Californian law on what constitutes a carcinogen is much stricter than that applied by the feds, and it prevails in CA. In the case of marijuana use, on the other hand, federal law is more restrictive than that which generally obtains in CA. The Federal narcs are generally too busy to go after individual pot smokers, but they have the legal right to do so.
Finally, some of us may not be aware that interested feds monitor sites like this, and they can subpoena all of the information that they want about you!
Obey the law! God bless America!