Hiring freelancers, on the other hand, is usually pretty simple, since none of those laws apply.

The main risk, say, as a Canadian, is if I visit a US-based client, then I can't be officially "working while in the US", but it's OK if I do it remotely.

I freelance for my local Canadian company, and if my company sends me to the US, then it's fine, because the client pays my company, who then pays me. The US company is not paying me directly.

Without a valid work visa, they might not even let you (Canadian going to us) into the country. We’ve had salesman turned back when doing training cuz the border guards thought a fee was involved. Sales support was ok.

Yep you have to be very careful in how you express your intent.

I had a friend turned back because he said "the client pays me", when he actually meant "the client pays my company, and my company pays me" (he had an incorporated 1-person company, not freelancer).

When I was purely freelancer, I would say "I'm going to see a client, but it's only social, I'm not paid". Customs would frown, but let me pass. I've even had an agent argue "so you have nothing else to do on a weekday, and your work-friend either?" to which I said "yep, we're computer nerds", and he let me pass.