Why are drugs manufactured abroad, so readily available on USA streets these days?

I used to purchase my asthma medication, from Canada, at a fraction of the cost (I was actually told to do it from my allergist) but they literally put a stop to it. The last package was opened, at customs, and I was given a warning.

I have no idea who is purchasing drugs from abroad and getting them delivered here? Maybe I can have their contact information? 😉