No that's not quite right, if you earn less than $10,000 in the US then you don't have to pay
taxes on your earnings in the US, provided you are directly employed by a Canadian employer while you're doing the work.
In addition if you are being sent by a Canadian employer to do certain things that are listed in NAFTA you don't need work authorization.
So for example, say you work for and are sent by Canadian company X to install a piece of equipment at US company Z, and the Canadian company pays you (and ordinarily would pay you) less than $10,000 total during the year for doing that work then you don't have to file a US tax return or pay US taxes on that money. Nor do you need work authorization as installations are generally exempt under NAFTA, and you just go in as a regular B-1 visitor for business.
Do a web search on "B-1 NAFTA entry" and look up the specific regulations on what you can do without work authorization.
Steve.