Flagging isn't expensive in the same way that upvoting content costs nothing. Flagging is a missed opportunity to use your voting power to earn something rather than to penalise something.
You're right that there are no rules about what is on the trending page and what can be upvoted by paid voting services. But if a product is being sold by a charlatan, has no scientific basis for the claims about its benefits and hasn't even been proven to be safe for consumption, then I greatly doubt anyone is going to be worried if the votes were pulled and flags were dished out at will.
I also hope that communities will help address the current problems we have with paid votes and the trending page. This type of shit doesn't need the type of exposure it's receiving here.