You didn't really get my meaning. People normally try to use the same app on all of their devices -- phones, tablets and computers. So, most of the users who use mixed platforms will choose the apps available on all of their devices, thus the majority of iTunes Radio users are those who own only Apple devices. Among the 8% iPhone owners (take feature phone owners into consideration), at most only 1/4 of them don't have a Windows computer or an Android tablet, if you multiply 8% to 1/4, that results in 2% in the end.
I see what you're getting at, but even assuming you're correct that people are likely to use the same service on all their devices, I would still wager that it's highly unlikely that your back-of-the-envelope estimate is correct.
Given that the lowest of any of the numbers is the Mac users, at 7%, that would be the baseline (assuming, of course, that everyone in the world has a PC, which isn't the case, but we're talking about people with a good enough internet connection to listen to internet radio here). But Mac users are almost certainly
not going to be a random cross-section of cell-phone purchasers; I would wager a large sum of money that they're drastically more likely to purchase an iPhone than the average Windows user. So with 15% of all smartphones being Apple in general (feature phones don't even count, since people aren't listening to music on them, and if someone
is using a non-radio and non-phone portable device to listen to music, it's almost certainly an iPod), you'd be starting at 1% of people being pure-Apple if it was randomly distributed (or higher on account of feature phone users with iPods, but no internet connection on those, so not relevant to the current discussion). Double the average iPhone purchasing incidence for Mac users would result in your 2% incidence, but I'd wager it's significantly more than double.
There's also the growing number of people--mostly outside the US, admittedly--who don't even own a "traditional" computer, so whatever smartphone (or tablet) they own
is their computing platform. Because a lot of them are in poorer countries, most are probably using Android, but there's also Japan, for example, where Macs are rare but PCs in general are
much less common than the US, and the iPhone just went from 15% market share to 35%, so there are probably a significant number of people for whom it's their primary computing platform.
And even setting all that aside, you're operating under the assumption that if somebody has a PC at home and an iPhone in their pocket they are automatically going to listen to Pandora because they're running it on their Windows machine, and acting as if iTunes Radio doesn't run on Windows. This is doubly mistaken; some percentage of them will never listen to music on their computer but will on their phone, and more importantly a lot of them are already using iTunes on their computer to manage music for their iPhone, so they have the option of using it for iTunes Radio on their PC anyway. No guarantee they will, but I wouldn't bet against it.
So even if every one of the iPhone users in the world has a PC at home, they still all have the option of using iTunes Radio on all their devices. It's only the Android users that don't.
Heck, I know a few Windows users who don't own
any Apple devices and still use iTunes to rip CDs or buy music.