Actually it isn't.
From a consumer point of view the buck stops with you, the retailer. You wern't forced into anything. You negotiated a contract to allow the sale of these products so it would be more precise to say that you support this otherwise you wouldn't have signed the dotted line. They could have held a gun to your head and you could have still refused...but you didn't.
Retailers/distributors of a product have to abide by several RULES. Some are imposed by the Manufacturer of the product, and others are by local Government regulations under International Trade agreements/limitations.
Simple example... Games such as Wolfenstein were/are prohibited to be sold and/or distributed within the Legal boundaries/jurisdiction of Germany. Is THAT a fault of the Manufacturer, the Retailer, or the Distributor?
Nope.
DVD Region-coding limits either by time/calendar or by outright prohibition the distribution of Film and Television.
In Australia [where there is no R-rating for PC games] some games are therefore prohibited by Australian Govenment Censorship Laws. [want an example?...last week I got into my head I'd like to get a copy of the PC game 'Starship Troopers' ....not apparently available in Australia anymore...but I found it on Amazon....but on my way to checkout was informed it was a prohibited distribution to AUS and could not be sent].
Stardock [via Impulse] is entirely free to choose between one of two options.....they are...
1. Facilitate Online purchase/distribution of a game/software under whatever pricing and/or conditions imposed by the product owner.
or
2. Not Facilitate purchase/distribution of that product at all.
There IS NO third option.
People can bitch and moan as much as they like, typically through naivete and/or mis-information but it will not change reality.