Winter, turns out, can get pretty cold. And boys will be boys, so eventually they'll whine and cry and make all sorts of awful noises if you allow them to freeze. Probably better to just keep the little snotbags warm and alive. And stylish.
When they set out to start making clothing, Fila realized it would be a fool's errand to try and make great clothes for everybody. So they limited themselves to two markets: athletes and young boys. This was no arbitrary decision; they methodically ruled out other groups. Young girls, for instance, are far too often dominated by the fashion senses of their overbearing stage moms. The lazy change clothing far too infrequently to be counted on as repeat customers. Nudists were right out.