That's loosely based on the threefold law:
Rule of Three / Threefold Law
A tenet held by some Wiccan practitioners and other followers of the occult; states that whatever energy is put out into the world by a person, that energy will be returned to that person three times.
Also just straight out Karma. If you take wrongly from someone (their money) something will be taken from you. Use your gift selfishly and things will go very wrong, very fast.
I believe a modest exchange of money for someone's time is not an evil thing if someone desires a reading. Modest being the key word here. I don't believe anyone's time is worth $500 and up for 45 minutes of a reading! I've actually seen readings advertised in the thousand dollar range and up.