It should be noted that the 5th Circuit Court of Appeals has ruled on several occasions that they have no problem with the use of drug sniffing dogs at internal immigration checkpoints as long as the primary purpose of the stop is immigration and the length of the detention isn't extended outside the time frame necessary to make reasonable immigration queries. This checkpoint took place within the jurisdiction of the 9th circuit however and the distinction the 5th circuit has made between primary and secondary checkpoint purposes is a distinction without a difference.

Indeed, if the intent of the U.S. Supreme Court was for drug checks to be allowed at immigration checkpoints, the court never would have stated the following in Indianapolis v Edmond:

"We have also upheld brief, suspicionless seizures of motorists at a fixed Border Patrol checkpoint designed to intercept illegal aliens, Martinez-Fuerte, supra, and at a sobriety checkpoint aimed at removing drunk drivers from the road, Michigan Dept. of State Police v. Sitz...In none of these cases, however, did we indicate approval of a checkpoint program whose primary purpose was to detect evidence of ordinary criminal wrongdoing."

Petitioners state that the Sitz and Martinez-Fuerte checkpoints had the same ultimate purpose of arresting those suspected of committing crimes. Securing the border and apprehending drunken drivers are law enforcement activities, and authorities employ arrests and criminal prosecutions to pursue these goals. But if this case were to rest at such a high level of generality, there would be little check on the authorities’ ability to construct roadblocks for almost any conceivable law enforcement purpose. The checkpoint program is also not justified by the severe and intractable nature of the drug problem. The gravity of the threat alone cannot be dispositive of questions concerning what means law enforcement may employ to pursue a given purpose. Rather, in determining whether individualized suspicion is required, the Court must consider the nature of the interests threatened and their connection to the particular law enforcement practices at issue.

or

"Our prior cases have limited significantly the reach of this congressional authorization, requiring probable cause for any vehicle search in the interior and reasonable suspicion for inquiry stops by roving patrols. Our holding today, approving routine stops for brief questioning is confined to permanent checkpoints. We understand, of course, that neither longstanding congressional authorization nor widely prevailing practice justifies a constitutional violation" - U.S. v Martinez-Fuerte

I'll be writing more about the current state of the law and why the Border Patrol has been getting away with using immigration checkpoints as a pretext for general law enforcement purposes in the near future but this should give you some idea of where I'll be coming from.

In the meantime, folks may want to give the recent Supreme Court case Arizona v. Gant a read to see how lower courts have misinterpreted higher court decisions in the past to give enforcers far more leeway to act than is constitutional or legal.