The idea is that continuing to make purchases adds plausibility to the framing.

You appear as if you are cooperating fully, and innocent (because a guilty person who knows they are being investigated stops doing the illegal thing).

If it was the housekeeper doing the crime, they wouldn't know bout the investigation so they would keep criming.

Since you don't know how long you've been monitored prior to learning of the investigation you don't want to suddenly change the routine either... Same reasoning as the first point.

The only real move you have is to hope hope the evidence points at the housekeeper and the frame-up tricks the investigators.

Im not saying i agree with any of the above, but it's the type of reasoning behind the actions.