Actually the marijuana being smuggled into the US would still be illegal, much as bootleg liquor is illegal. So those interdiction efforts would still continue. Also as said, when patrolling, the officers are not specific as to drugs, just they are looking for smuggling, be it drugs, contraband goods, or even people.
Marijuana is an odd case as far as drugs go. It does have medical benefits, the addiction rate is lower than alcohol, most users are not reduced to crime to pay for it. However, it is a mind altering substance and can cause harm to the human body if abused. It definitely does not have all the dangers of alcohol, which is legal. If pot were legal, the price might be even lower with taxes, because delivery risk and cost would be lower. I don't know how I feel about legalizing it, but there is a lot of misinformation out there from both sides of the issue.
This is one of those issues that turn politics on its' head. Those that argue for smaller government, less government control, and the right of individulas to make thier own choices are usually the same ones arguing against legalising pot. Funny world.