When you listen to streaming radio you are pulling down data from the network the entire time, as you need it. This means you're receiving little bits all the time in real time. The weaker the reception, the more power it uses to stay with the signal constantly.
When you listen to a podcast, it's not real time, it's a file. You're just downloading a file and playing it back. So the data downloads as fast as your connection allows, and the faster the connection (e.g. LTE vs 3G), the sooner the podcast file is finished downloading. This is important, because once the podcast file is downloaded there is no more need for the cell signal for playing back the podcast. The radio could completely power down and you can still listen to the podcast because it's now a downloaded file stored on the phone itself. On an LTE connection an entire podcast could download in less than a minute and then the radio can power down. But if it was a radio stream, of course the radio could not quit after just one minute; it would have to maintain a high enough power level to continue receiving the cell signal as long as you are listening.
That's why streaming audio/video uses a lot more battery than an audio/video podcast.