An employer in the US is bound by US laws on whether the OP is an employee or contractor, no matter where the OP is. (If the OP was defined as an employee then many laws like the ADA would not apply to them, assuming they never travelled to the US office, but Canadian employment laws would, like required notice periods - it's a very complex setup and many employers just won't do it, some smaller companies do it without actually understanding what they're doing.)
> Also, will I need a work visa if I’m to be employed by a US company, even if I work from Canada?