Factual error: Phoebe is going to be burned at the stake for witchcraft like they're starting the Salem witch trials again, but witches weren't burned, they were hanged.
Factual error: During the ritual, we can see that the snake that is magically transformed into a man is a milk snake. But it's not poisonous, it's a colubrid. The creators apparently wanted to use its resemblance to a poisonous coral snake, which is an elapid. (00:02:35)
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.