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That's no longer a problem because it appears all ads in the content network are now disabled when you display pages with AdSense via Google cache links.
Now the fact that cached pages used to display off topic ads was silly because Google knew where the pages came from so there was no rhyme or reason that AdSense would go off topic whatsoever unless there was an internal disconnect between Google Search and AdSense. This made no sense anyway because Google claims both Googlebot and AdSense Mediabot share the same cache of the pages, so it never made sense because both the webmasters and Google were losing money from this ridiculous situation.
Now, the pages with AdSense display nothing whatsoever so it's a complete loss for all involved.
No PSA, no off target ads, just a big blank spot making us all look stupid.
There is one caveat to this in that I've seen a couple of premium publisher sites still displaying AdSense ads in the cache, but none of the 30+ publisher sites I tested for regular AdSense ads showed anything at all in Google cache.
Personally, I'd prefer the ads enabled when cached pages are viewed and the same ads properly displayed that would be seen on the same page when in context of the website, so everyone involved could make some money, but that's no longer an option.
What's the reason for this current change?
Could it be a simple technical malfunction?
Is it possible that AdSense in cache was disabled so that the cache pages are truly "fair use" and Google isn't monetizing them?
Could it be a sign of the times [webmasterworld.com] and with some belt tightening that Google doesn't want to share revenue when the visitors are on their site? Are they trying to get 100% of the PPC revenue when the visitor clicks BACK out of the cache?
No clue, purely tossing ideas around.
I'm not sure of the reason, but it's a huge change without as much as a peep about it from anyone anywhere!