Yes they do. LAPD catches drug dealers all the time too. Should they go down to San Ysidro, sit a mile from the border and stop and search everyone walking down the street? It's all about jurisdiction. CBP has the border. Other agencies cover the US. The CBP has decided that the border isn't really the border, it's an invisible line 50 miles or so north of the border. That's the rub. If DEA or local LEs want to set up a dragnet on I-8, let them get authorization to do so from the courts. Having CBP set up a checkpoint well within US borders is a circumvention of that.

Lets say CBP finds drugs on you on I-8. What if you grew them in California and are driving them to Phoenix? You haven't crossed an international border, so thus Customs has nothing to do with it, and since you didn't cross a border, there's no Border to be Protected.

If you agree that CBP can setup checkpoints inside US territory, and check for anything they feel like checking, then you're saying that CBP can de facto operate well outside the scope of their assigned missions on anything within this new Frontier Buffer Zone. They could set up at a mall in San Diego and ask everyone that passes what their citizenship is. They could pull you over in downtown McAllen Texas and ask your citizenship.

The 50-mile checkpoints were supposedly to search for illegals. Now, it appears they're looking for drugs. What's next? That's the point, there's a constant pushing on the boundaries of jurisdiction and scope of responsibility, not just by CBP but by a lot of LEO and pseudo-LEO agencies. You seem to say you're ok with CBP catching drugs in the middle of Southern Arizona or New Mexico. My question is, how is that in any reasonable sense a Border Protection function?