I have the same question. I suppose ads billing might have transactional requirements, but that's separate from search itself.

I worked at Bing/Yahoo.

There are two mechanisms helping keep search up when considering ads:

-> Search engine pages can be served w/ no ads, or even no personalization if components are down or lagging

-> Ad logs can be lost if necessary, since we'd under-bill in that case, which would be our own problem. That happened a small handful of times while I was there.

The billing system is indeed transactional, but has less need for 100% up-time and 0 latency in comparison to search. Not nearly as big of a deal if there's an hour delay on billing ads as compared to search being down for an hour.