Personally, I would love TV shows on Blu-ray. My Land of the Giants DVD set is 14 discs across two box sets, where it could easily be released as a single case 3 BD package. Similarly, my Tom Baker Doctor Who DVDs span 15 individual cases, once again to be easily placed on 3 BDs in a single case. In fact I held off for the longest time on buying the Dr. Who DVDs precisely because I had (faint) hope that they'd be released on BD (in SD of course, as they're sourced from video tape) at some point.
But I have to agree with the requirements of the Blu-ray Disc Association, it cheapens the Blu-ray brand and would cause confusion. Here in Australia, we had an early release of Pulp Fiction, prior to the more recent proper high quality international release, which was simply the DVD upscaled to 1080i and slapped onto a Blu-ray disc. It was so bad, I literally threw it into the bin after the first viewing. It leaves a bad taste in the consumer's mouth if they get SD content on an HD format, because like it or not, a lot of people will not do the research, will not read the back of the case or the seller's listing in detail, and will expect remastered shows in HD when buying on Blu.
I'm not even sure how much demand there would be for SD content on BD. I know it's hard for some of us to understand, but I suspect a very large number of consumers are still against the concept of Blu-ray entirely. They already have a DVD collection and don't want to rebuy their movies in HD, so they resist any move to Blu for any reason, even for SD content. I know people who won't buy a Blu-ray player even though they're dirt cheap now. It makes no sense, but it's happening. The vast majority of disc sales are on DVD, so companies will just release SD content on DVD with very little risk of customer backlash.