Protection from Malicious Intent
Google does do occasional audits. There were apparently some wallpaper apps recently that were doing nefarious stuff and were pulled from the Market and any phones they'd been installed on.

It's imperative that users pay attention to the permissions that are being requested (Why would a wallpaper need to know "Phone state" or my Contacts?) and simply not install anything that looks wonky. Pay attention to the ratings and the comments and, if something looks too good to be true, it probably is.

Everybody wants everything for free. The price of that is that your marketing demographics are being sold to anyone willing to pay for them. I don't begrudge developers the ability to make a profit.