I have seen this too and thought the same, but here's a couple added subtleties that I think need adding to the equation.
An old teacher of mine once said you should only study martial arts from a teacher who does not have a side job. What I think he meant was that you want someone to teach you who is serious and has put the time and effort needed into their practice to indeed be worthy as a teacher.
But here's the thing. I have found, and this is not limited to martial arts but many other artistic fields also, that if your livelihood is purely dependent on this income, on some level you lose your independence when you attach money to art. At the harshest level, you are prostituting yourself to the whims and wants of your clientele so you can keep them coming back and paying you.
An artist friend of mine told me once that if she wanted to earn more money, she'd just paint cats and dogs, because they are a guaranteed sale. To paint what she really wants, she has to make a conscious decision to risk NOT making that money.
You might say, well why not paint what people want? But I suspect you know that this road leads only to generic reproduction with no room for confrontational or innovative art. Substitute 'Martial Artist' for 'Fine Artist' and you might also be able to see how corrosive this path might be - Teachers only teaching 'feel good' material, and stringing their students along as far as possible with promises of the 'secret stuff' to come.
I believe this is why those that say connecting money to art is a bad thing.
They are missing what is probably the biggest point in this whole thing:
THIS DOES NOT EQUATE TO THE IDEA THAT HIGHLY SKILLED TEACHERS HAVE NO WORTH.
I'll say it again
SAYING THAT TEACHING FOR MONEY CAN CORRUPT THE MATERIAL IS NOT THE SAME THING AS SAYING THAT SKILLED TEACHERS HAVE NO WORTH.
If I have spent years and years of time and effort to understand what I know and I'm good at it, why should I give it to you?
What is YOUR part in all this? What is the student's role?
See, the student also has a responsibility. Just as I feel I have a responsibility to pass on what I know, the student has a responsibility to value the teacher. To be happy to help them financially, as well as put in their own time and effort to gain the knowledge they are being given.
Really it is the student's job to OFFER to pay for the teacher's time, no? Not the teacher's job to ask for it. Where did that get lost?
It is also the student's responsibility to value the knowledge, separate from valuing the teacher. This knowledge may well be arcane, and it will not help with fixing the leak in your bathroom, but it is part of the human experience, a very old and deep aspect of it. It should have it's place beside all the other skills and fields of study that help us connect with being alive on this planet.
So think on this, and remember, respect and responsibility go both ways. If that's not clear, just ask yourself - Why would this teacher spend they valuable time teaching me? Why would they want to give me what they spent so many years of effort to learn?
And, no, the answer is not just 'money', but it's also not just 'because they should', or 'because I'm a nice person'.