Absolutely, outraged - totally agree that it's about market rate. And the 'not running on a charitable basis' cuts both ways. You can't 'underpay' a nanny, since you've both agreed to that rate. If she could get a better overall combination of pay and conditions in another job on the market, then she would.
But the key thing is overall combination of pay and conditions, and I've often seen the argument about cleaners rates, where people fixate on a number rather than think through how those numbers genuinely compare.
If you think about how a market rate comes about, it means that x number of parents have weighed up the pros and cons of having a nanny over other childcare, and decided how much it's worth to them. And the same number (to simplify a bit) of people have decided that nannying is a job they would prefer to do - at that rate - than another job.
If more parents wanted nannies (maybe because the cost of nurseries went up, or salaries went up generally so more people could afford it), then there wouldn't be enough nannies and the rate would go up as people competed for them, eventually attracting more people into nannying and establishing a new balance at a higher rate.
If less parents wanted nannies (nursery costs or general wages went down, or nursery provision improved) then the surplus of nannies would drive down rates to a new balance in the same way.
In exactly the same way, if some other job (teaching - or cleaning!) was overall a better option for someone - when they think about all the factors including pay (whole package), how much they want to do that job, cost and time required for training, job certainty/uncertainty - then they would do that job instead! That would reduce the supply of nannies, and push rates to a higher point as parents competed for them.
Finally, if other jobs became less attractive overall than nannying (eg increasing paperwork in teaching) then people will eventually move into nannying pushing prices down.
And when I talk about parents competing, I simply mean that for some parents the threshold for other childcare (or staying at home) will be lower than others. When the market is pushing up, then those parents will drop out until balance is achieved.
So let's not get upset about people asking what going rates are. It's perfectly reasonable both for parents and nannies to try to get information about where that balance lies. There are no 'unfair' rates, simply people separately deciding what works for them, and hopefully making an agreement when those interests match.