You say that's a rule of magic like it's a natural law, and in fact it is not; that's a rule of some traditions. You go to Hong Kong or even to Chinatown and there will be loads of people who are perfectly willing to cast you a spell for money. Heck, open up the back of any tabloid.

I don't believe there is anything wrong with doing things for money that you wouldn't have an ethical problem with doing in the first place; the reason the lines get so messed up with people who cast curses and the like for cold hard cash is, first of all, hello, curses, and second of all, if you're being paid you are only hearing one side of the story so you may or may not be doing enough research before you take a case.

I've been paid for magic; I gave readings and tried to solve problems for money for years, particularly when I was in graduate school and the terms of my assistantship were such that I couldn't have outside paid employment but I didn't make enough money to pay for all the necessities. It didn't ruin MY life. I was a phone psychic, too.

This is one of those things that the whitelighters, usually privileged people who do what they do as a hobby or a spiritual practise, would like people to believe. I can well believe it made it into a TV show like Charmed. But um. A short stroll down any number of Chinatown or New Orleans neighbourhoods should be sufficient to disabuse you of the notion.