People who do podcasts as a living or companies where podcasts are their primary revenue source need to make money somehow.
Take How Stuff Works for instance. They have some great informational podcasts like "Stuff you Should Know", "Stuff to Blow your Mind", "Stuff they don't want you to know", and "Daniel and Jorge Explain the Universe". These podcasts cover very interesting, and sometimes, very important topics. How stuff works makes their money from selling ad reads in their podcasts and ad space on their blog. They are providing a valuable service and paying people to literally do nothing but research and produce podcasts. These people need to get paid.
My point is, while nobody actually likes ads, the model which podcasts employ is at least tolerable; especially when compared to something like ads on Youtube.