The counterfeit drug industry is a $200 billion a year business, and it is estimated that 10% of all drugs are counterfeit. And yes, many of the drugs that are re-imported into the US are counterfeit. So that's reason #1.
The other reason it's illegal is because countries like Canada impose price controls on American manufacturers and threaten their patents if they don't what they're told. Most people who want to buy re-imported drugs that government "negotiated" at the point of a gun don't realize that they are benefiting from socialism. Many don't care as long as they get the lower cost. To hell with everyone else. Others, however, do care because they know that without profit, there is no innovation.
I'd like to see my kids live in a world where drug resistant bacteria, cancer, Parkinsons, Malaria, MS, MD, ALS and on and on are no longer a threat. The sure way to kill that dream is to control the profits of the pharmaceutical industry.