I'm not an immigration attorney, but I have made trips to Canada to find clients, and this is my understanding of the laws.
If you are a Canadian citizen or permanent resident, you do not need a visa to come to the United States as long as you will not actually be working and receiving payment from U.S. businesses or employers while in the U.S. For example, you can visit the U.S. to attend trade shows, meetings, conferences, etc. without a visa.
Here's a publication from the U.S. State Department that may help: http://travel.state.gov/content/dam/visas/BusinessVisa%20Purpose%20Listings%20March%202014%20flier.pdf
This is not legal advise, and I don't play a lawyer on TV but I would say as a contractor all work worldwide is open. Your not moving to the US your just taking business from their. There maybe a question of where you pay tax at some point so set up a Hong Kong Corporation that is near impossible to sue if your going to be dealing with goods internationally. And why restrict yourself to the USA only, hoped this helped call if you want to discuss it further, thanks...Ken Queen