Apple do appear to me to enforce Retail Price Maintenance. As do some others, such as Montblanc.
How they do it is about mechanism, not principle.
if they only offer retailers low margins, that is one way, but it is not the only way. Their staff get a 30% discount, as do many retail staff in high value items, (think jewellery - staff discounts of 35% and 40 % exist today in the UK High Street). So Apple can afford to lower their prices. They do not want to - they simply make ginormous profits. So do Microsoft. Just cos one does, no need for the other to follow by ripping off citizens.