honestly I wasn't even aware of the fact that the s3 has wireless charging. Nokia is advertising this feature heavily thus should include 1 charging pad in the box. That's my opinion
I don't think they can afford to for everyone. But I see your point, if they want wireless charging to take off, they have to get wireless chargers out to people. What I think they should do is some sort of deal like if you're one of those who preordered, get 50% off the wireless charging mat. Or for a limited time, if you buy a phone and those Purity headphones, we'll throw in the wireless charging mat for free.
That way, they'd hopefully be encouraging preorders and boosting early sales. Also, they're not thinning their profits like they would if they included it in every box (because some people even said they think wireless charging is useless and they'd never use it, so why give them something they wouldn't use?). Plus, what happens next year if you buy a Lumia 940 (or whatever they call it)? Okay, you have two, one for home one for work? Then what about next year? And the year after that? You're going to have a desk drawer full of unused wireless chargers then, and that's wasted money (for Nokia). That money could have gone to more R&D or something, shrinking down that 41MP PureView sensor.
So yeah, I think some sort of accessory deal or promotion is the best option.