Ad serving is a (very) low-margin business, and only profitable with huge impression counts. Apple services (think iCloud storage plans) are high margin, with comparatively few opportunities to serve ads. There would be zero point to injecting ads into service contexts (as opposed to something like App Store views).

Do you have some examples of services going from fee-for-service to fee-and-ads-for-service? I am coming up blank. The closest that I can think of are areas that had been as based but then added fees as well, such as TV.