No, they don't think like that. They just don't want advertisers to financially support extremist content.
Maybe it's that TV itself is mostly bland and ad friendly, non-controversial for that very reason. Content that is obviously for entertainment purposes is cool to put ads on. TV had few channels and programs that weren't for the general population, so you wouldn't have The KKK Weekly or Cool Communism that advertisers would have to avoid.
That's different on YouTube, where the content is much more heterogeneous, where you have super-optimized, ad-friendly influencers with millions of subs who take great care not to offend (and are therefore great to throw ads at) next to fringe political ideas and people with mental health issues presenting their world view.
I believe funding is often considered as an endorsement, and ads are the primary source of funding for most media companies. Similarly, you'll often see disclaimers on Twitter "retweet != endorsement" because people tend to understand it as such.
I believe that the bigger deal is subconscious association. I have no idea whether it's true, but putting McDonald's ads next to a documentary on factory farming seems unwise.
I imagine TV before I was born was a little more like this since you could't just buy generic ad space - you needed to deal with the producers of a show specifically to get into their ad slots - but for TV when I was growing up, ads were ads. They'd target your age group (ads for kids toys during breakfast cartoons), but it just never occurred to me that anyone would think it was an endorsement of anything.
This is such a confusing concept to me. Do people really think like that? I know they do now, because it's a self-reinforcing concept. Yelling loudly that ads are an endorsement of content makes it true - since if you're an advertiser and you know that attitude is out there, and you still run an ad, that's now a conscious decision and everybody knows it. But it seems like this came about artificially, via activism, rather than people naturally making the link that ads imply endorsement.