To clarify this thread even further, because I had to do a lot of research before our station started streaming.
It was AFTRA that pitched a fit about not being fairly compensated. The ad agencies told the radio stations to stop streaming the ads or they will have to pay what the voice over people wanted. However, they did eventually settle on a fair rate. In the meantime, however, technology made it easier to block out the ads. Some streaming providers will block the ads for you, in addition new automation software, such as Google's Scott Studios can substitute ads going to the stream.
In larger markets many of the voice talents belong to AFTRA, or receive ads from agencies that have AFTRA members voice spots. It's a lot easier to not deal with the hassle of weeding out which spot you can and can't play. So the vast majority of the large markets block their ads.
Smaller markets, like where I am, doesn't worry so much about that. Most of our ads are voiced in-house or are flagged by the network as not suitable for stream if need be.