Continuity mistake: Piper does not light the candles before she summons Grams, but in the episode 'PreWitched' (when Grams summons Patty) we see that the candles must be kept alight to maintain the connection with the dead. (00:18:00 - 00:33:30)
Continuity mistake: After Phoebe has her premonition about Paige at Prue's funeral, demonic bounty hunters show up to "eliminate" Belthazor. We see the funeral hall empty where seconds before Phoebe's premonition, there was a line of people waiting to give their condolences to the girls, Victor, Leo and Cole. (00:12:20 - 00:13:10)
Continuity mistake: During Prue's funeral when the demonic bounty hunters attack, one gets thrown into some chairs, which knocks some down. When we see the chairs again in another shot the ones left standing and the ones knocked down have changed. (00:13:25)
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.