If you're making a mobile app, or especially something for tablets, in a lot of cases most of your users WILL be using Apple products even if it's not specific to iOS.
Market share? It's been shown that even though a lot of "android" devices may be out there, the majority of use and engagement from mobile platforms is still coming from iOS.
For many markets (probably globally too?), there's no single smartphone that's more popular than the latest iPhone. So if you're only going to put one device in a photo, that'll probably be it.
And if you look only at the demographics you're interested in, it's likely that iOS will be at or near the top anyway.
