Charmed has inconsistencies. I know it so you don’t have to tell me and I’m not here to debate about how the rule was imposed but I want to talk about a theory of mine. I believe the personal gain rule was limited to use of spells and not powers or potions. Throughout the series, the sisters use the powers to do whatever they like entertain a fussy baby (S2), clean the attic (S1), freeze people who invade their space (S6), orb wherever whenever (Most Seasons) and there aren’t any consequences because of this. Personal gain is brought up but nothing happens. Even Leo used his levitation to do house work. Piper & Phoebe used potions to turn people who annoyed them into animals with no adverse results but when they use spells for selfish reasons there is some backlash. The only times their powers came with consequences was when used her premonitions to win the lottery before her ticket was erased and when she was forcing herself to have visions but those weren’t really issues.

Sub-thoery: I believe the Elders created this rule instead being that magic itself doesn’t allow this, like Leo’s healing, because Phoebe had to punished for forcing premonitions rather magic doing it. Even when Phoebe’s ticket was erased didn’t seem like a consequence as much as someone telling her “no, you can’t do that.”