One thing everybody seems to forget:
Apple does not author most of the podcasts. With the exception of a small number of podcasts (like the
iTunes New Music Tuesday one), they are authored by independent third parties. The iTMS simply acts as a large search engine. When you subscribe, the files are downloaded from third-party web sites.
If one of those third parties wants to insert an ad, nobody is going to stop them. And nobody should. If you don't like it, complain to the author or unsubscribe.
If you think Apple is going to download a third-party podcast, edit it to insert their own ads, and then feed the modified version to customers, think again. Such a process would cost a fortune (far more than the ad revenue could ever produce), it would create a huge disgruntled user-base, would be illegal (violation of copyright) and it wouldn't work (because you could still download the original files from the original source.)
So let's drop all the paranoid conspiracy theories already, OK?