I think the main reason Apple are giving it away for free is the same reason they always update iOS7 for free - they're primarily making their money out of devices - Macbooks, iPhones, iPads etc. so they can afford to give it away for free. Plus, it keeps us happy and wanting more from them, and makes them look good - whilst with Windows its still possible to buy a PC with an older version (i.e. 7, which in this case isn't a bad thing but lets ignore that for a second), you'd have to pay £60 upwards for the latest version on top of the cost of your PC, Apple are basically saying "buy the computer and we'll look after you, kid".
Also, might I point out its possible to buy Windows devices for Apple prices, with similar specs? Apple just don't do the lower end of the market - Toshiba etc. when making their computers are the equivalent of Samsung, trying to fill every niche. Apple meanwhile is simply trying to conquer the top end. And carve out new niches, although they haven't really done that since the iPad.