re: Is it legal for dental hygienists to open a teeth cleaning practice?

Let's use your $80 fee and not credit the hygienist for the bitewings or any part of the exam fee - though we both know that without the cleaning, most patients aren't scheduling an exam/x-ray on a routine basis unless they are having some indication of a problem. But let's assume the hygienist only gets credited with the actual $80 cleaning. We'll also assume that of the 12 patients scheduled, one fails to show in the morning and one fails in the evening. So 10 patients x $80 = $800/day gross.
The only significant overhead is the hygienist's salary - in an area where the cleaning fee is only $80, a generous salary would be $275. We're now left with $525/day.
There is virtually no other overhead. The front desk person scheduling the patients, making confirmation calls, filing insurance or collecting fees still has to be there all day whether you have a hygienist or not. Her salary does not go up because she does this for the hygiene patients. The rent for that extra space for that additional chair is insignificant. The electric bill for the extra light on in that hygiene room is insignificant. All your other overhead remains virtually unchanged.
Of course, there will be some minimal supplies needed. A bib for the patient. Some gauze, prophy paste, a toothbrush to send home with them, etc. But let's overstate this and call it $125/day in supplies and the other overhead costs that we know didn't really go up. We're still left with $400 straight profit each day the hygienist is in your office.
How is this a loss leader?