I'm trying to find a good analogy. Say we're in the early NES days (no online stores, just physical). Nintendo gets a margin on games, because presumably game developers have to pay them for dev kits or some such to make an NES game. The game devs (distributors, publishers, whatever) then sell the game to the stores for some price that gives them a margin. Then the stores add their margin on and sell it to customers. Three margins in there, from there different businesses that have competitors, so they try to keep their margins competitive.
Now let's say Nintendo made a deal with Toys R Us such that NES games could only be sold there, no other stores. Assuming that didn't destroy demand for NES games, Toy's R Us could then ramp up their margins to whatever they want, because there's no other option to consumers. Unfair advantage? Stifling competition?
With Sony, they own the store, and have ensured there's only one. So they get to leverage the same advantage as above. Is it unfair?
That's basically claiming business shouldn't be able to set their price if their margin is deemed unfair... which is a pretty daring claim. While regulation on pricing could be interesting it's not exactly a judge job and it would be a bit counter current with how the world economy currently work.
As for the whole "closed ecosystem" part this have been debated thousand of time already, nothing new here except I'm not sure it was attempted in UK yet.