Many smaller US companies don't have Canadian subsidiaries. In those cases they use a PEO (Professional Employer Organization.) The PEO holds your paper and is your employer of record but all they do is provide benefits and pass the payment through to you. I've worked for a couple of PEOs. The latest one that I worked for was called Global Upside. The company I am working for now converted us from the PEO to a Canadian subsidiary and ADP just over a year ago. It saved them a lot of money so they took some of that money and increased our benefits.

It's also really important to know your rights. My employer's PEO insisted on a pre-employment drug test. I explained that pre-employment drug tests were illegal in Canada for most employees. They insisted that I could not be hired without one. I sent them and my employer the relevant section of the law and a letter from an employment lawyer and my employer told them to fuck off. Now Canadian employees are no longer drug tested. They also have unlimited vacation which is illegal in Canada. So Canadian employees have unlimited vacation, minimum 4 weeks.