Love Hurts - S1-E21
Continuity mistake: In this episode Leo specifically tells Prue, Piper and Phoebe not to touch the Darklighters arrow because it is poisonous however in future episodes/seasons, the Darklighter arrows are only poisonous to Whitelighters. (00:06:30)
Love Hurts - S1-E21
Continuity mistake: When Prue shoots the Darklighter arrow at Alec, he orbs out and the arrow sticks into the wall behind him. In the next shot, the arrow has disappeared. (00:25:45)
Love Hurts - S1-E21
Revealing mistake: After Piper saves Leo, they meet with Phoebe and Daisy downstairs near the living room. Before the darklighter appears, smoke from a fog machine can be seen at the left of the shot to magically transition to the computerized effects. (00:33:45)
Love Hurts - S1-E21
Continuity mistake: Leo dies and then Piper heals him. In all future episodes it's made very clear that Leo's powers can't heal the dead.
Answer: That was a mistake in the series.
Also, the episode is called 'Babies First Demon' and this alone tells us that it's Wyatt's first time in the field (so to speak) and as the sisters say, they've done a good job of clearing up their own magical messes so far. Nobody really knows how advanced Wyatt's powers are, or will become. That's why the cleaners appear, to nip things in the bud. The Cleaners didn't want to chance that the sisters could clean Wyatt's magic up, just incase they couldn't. Therefore intervening themselves, just in case.