Financial reports aren't as strict as you might believe them to be. There are many widely accepted tactics utilized to keep certain things secret or just in general to paint a better picture. You can report on a deal with another company, but not specify every detail of that deal. Nobody would ever notice unless you're doing something egregious as suddenly having a 500% increase in "Other" revenue, or in the case of this situation being specifically investigated.

Not specifically this but for its part Google typically bundles these under “Traffic acquisition cost”.

This is the amount of money Google pays to third parties to get traffic into its services.

Many companies have a growing category on their income statement just called 'services'. They don't need to explain what services were bought or sold. Services is often where data deals land. I remember helping Microsoft setup a $1B contract for services with the DOD, but they wouldn't tell me any details. I can only use my imagination.

It isn't a major part of Google's costs, nor a major part of Apple's revenue.

Subsidies from apps, while they are usually bundled apps, which Apple doesn't do, are commonplace in hardware vendors' income. For PC makers, it is a more significant part of their business model.

I believe Apple lumps it into one of their “other” categories that includes all sorts of other things.

I mean is it? We're reading about it on HN.