Another category is by daypart. The commercial can run anytime between specific times.
The least expensive is what is called “run of schedule. The network can place the commercial anywhere it wants.
Only in the first instance can the advertiser be considered to sponsor a given show. Advertisers can, in some instances, if the network is cooperative, specify a program which they do not want their advertisement to appear.
We have to be careful in assigning blame.
Since these companies are advertisers for CBS, it is secondary whether they are specific sponsors of Letterman. As you say they are being allocated to him, and thus can demand to be dissociated from his show.
Protesting to these companies that use this "run of schedule" advertising, targets CBS and signals the companies to dissociate themselves from Letterman's show (which they can do anytime).
The block of advertisements just before the top of the hour are all Local commercials, sold by and run by the television station you are watching.
During the show itself, the commercial blocks are shared by the network and the local station.
The first 1:30 or 2 minutes are network commercials. The rest of the 4:00 minute commercial block are local commercials.
Incidentally, the same rules apply as above. Local stations follow the same general rules for commercial placement and the resulting costs as the network
Also, commercial breaks usually contain several commercials from different advertisers, so how am I to know which are sponsors or not.
How does a person find the disconnect between knocked-up teen jokes & the candy bar commercial that follows it? Maybe imagining the rape of a 14 year old girl by a promiscuous celebrity is more pleasurable with a bag of M&Ms? Is Lifelock some kind of a chastity belt?
I guarantee you that if a major advertiser called CBS & said they do not wish to be associated with the Letterman show, their wishes would be granted. Therefore, if the advertisers fail to make that call, then they are giving tacit approval to Letterman, be it real or perceived.
Letterman's sponsors need to reflect the show, so I suggest condom commercials, bathhouse commercials, & public service announcements re. the spread of STDs.