Just because it's the norm doesn't mean it's not excessive. In contrast, Apple's implementation of a 30% cut is even worse, since with an iPhone you can't just install an app from another source (and even when you can in the case of the EU, there are recurring costs for doing so). Since Steam accounts for the majority of PC video game sales, with AAA titles only not releasing on it when they have a clear financial motive not to, Valve's use of a price parity clause effectively makes it the arbiter of what the industry standard markup on PC should be.