Substitute ads and or content is *usually* because of licensing, or
lack of...

Chevy (or whomever) may have only paid for their ad for a spot ONLY on
broadcast radio (over the airwaves only) and not on the webcast.

Another factor is royalties paid to artists who do the commercials, who
have contracts that specify fees paid to the artist depending to the
distribution...broadcast or webcast...

This can also affect the music beside the commercial spots...

PSA's also make a station look good at license renewal time...

And that's a wrap...