It's possible that there's a shortage of AdWords buyers for those topics who have chosen to allow content ads.
I just did a spot check of my site, and I'm seeing the usual targeted ads:
- Ads for various Portugal hotels on a page about ruins in Portugal
- Ads for barge cruises on a page about barge cruises and canal boats
- Ads for car rentals (three of them specifically mentioning European car rentals) on a page about car travel in Europe
There are a few exceptions, of course, but they're mostly because Google misinterpreted the topic of the page, not because it's displaying generic "site theme" ads:
- On a page about Berlin sightseeing and excursions, I'm seeing ads for San Francisco day tours and London tours in addition to an ad for a company that sells tours in various European cities.
- On a page about luxury travel, three of the ads are on-topic, but the fourth is for luxury hotels in Bali (which is a long way from Europe, the region that I cover).
If I had to choose between ads for San Francisco day tours or Bali hotels and generic "site theme" ads about European travel, I'd choose the latter. With luck, AdSense will either improve its topic-matching algorithm or allow publishers more control over default advertising themes and specific keywords. But in the meantime, AdSense is working pretty well for my site on the whole if not in every specific case. (Mind you, I'm in a category that attracts a lot of advertisers, and that could be why I haven't yet encountered a plethora of generic "site theme" ads or public service ads on my pages.)