The money is certainly very good compared to what they would make working in a restaurant or supermarket given their lack of English.
Your math is correct. The majority would make an average of $6,000/month - $72,000 per year. Mostly tax free - I say mostly because they still declare some income to file taxes. Some owners will issue a T4A. To move ahead in life and to achieve their goal of owning property, they have to show income. These days even a condo is $700,000 plus. Downpayment of at least $200,000 is required if not more if their income do not qualify for a mortgage. Some of them even need a guarantor to buy a car. So yes, they do declare some income if they want to own property.
Out of $72,000 one has to pay living expenses. Rent, children needs etc. If they do not have PR status and are here on a open work permit, their children university fees is $25,000 - $30,000 per year as a foreign student. Most are single Moms. Not much left after expenses. Even if they get PR status - after and arranged marriage that cost close to $40,000, their yearly expenses would be close to $50,000 if not more.
A lot of them do not choose the profession. The profession chose them. They chose to do this out of necessity. They chose to support themselves and not rely on handouts.
And NO, spa owners do not take a cut of their earnings.