the laws says it must be anti-competitive. what steam does is NOT as it does not harm the market. quite opposite as it prevents steam from destroying retailers who could not lower their prices to steam level (there are a lot of reasons why a boxed game costs up to 110 american dollars in a shop)
and what deano said is incorrect: you can charge what you want for whatever you sell as long as you don't control the market.
sellers forcing resallers to sell at set price is illegal.
And i bought quite few times a magazine, or newspaper at lower price. sometimes it was damaged, sometimes it was a bit old and sometimes they were running a promotion (get a coffee and a paper is 50% off)