I think part of this comes from the abstractions used for tablets vs. "traditional" computers. (With the surface being viewed as a tablet by most consumers, vs. the air being seen as a traditional laptop).

In tablets, the trend seems to be to abstract away/hide the OS as much as possible – it isn't something that runs on the machine, it is the machine – if that makes sense. Compare this to the laptop world where we're all engrained to conceive of the OS as something that is installed on the machine and in most/all cases is able to be swapped out (insert plug for your favorite Linux distro here, etc.)

So, given that, most consumers willingly accept the space the OS takes up as a given on the laptop yet the same people see the listing the tablet storage including the space required for the OS as disingenuous.

When you want to abstract away something, if your abstraction leaks, it usually hampers the user experience or user perception of the offering – as this case illustrates (IMHO).