As a US employer, do I need to try hiring an American worker first before I can hire a remote Canadian worker?
The short answer is no because you are not hiring a worker on US soil.
This situation is akin to a US company contracting out an Indian company to perform some of its operations in India and is fully legal. The US company is not required to first try hiring American workers in America before they are allowed to use an Indian contractor with workers in India. This logic also applies to hiring a remote worker in Canada via a foreign PEO agency. In the eyes of the US Department of Labor, this is purely an international business transaction that that does not involve American workers on US soil.
The only time a US company needs to vet American workers first is when they want to hire a worker to work inside the USA. In such situations, they must show the Department of Labor that they have tried to hire an American first, but couldn’t and hence the reason they want to petition a foreigner to be brought into the USA.