It wasn't a smart answer, it's the truth. Those vendors paid for advertising to be used on the entire site, not just parts. And some people use both programs not just one, so it makes it worth the paying vendor's while to have their advertising cover all the bases. If a poser vendor pays for the advertising they will get the same treatment as well. Your example about radio stations is invalid because there are many types of contents available here, not just one. And even then, even on my subscription of SirrusXM they advertise content provided on their other channels, so they're going to let me know that they have a comedy station while I'm listening to my dance music. They announce their Christmas music channel over all their stations this time of year to to cover all their bases. The same goes with internet radio stations.

The actual name for this type of advertising is called "cross-promotion".