This goes for every place though. Ultimately someone decides the price somewhere, even on Steam. I'm not sure who sets the price on Steam though, if it is Valve themself who take the loss or if it is the publishers who price their PC games cheaper because they want to reach more people on PC. Does anyone know?
Monopoly is also mostly when we talk about one group of product, like if Gamestop was the only store that was allowed to sell games. But with stuff like PS3, there are competitors out there, like Xbox 360, Wii and PC etc. So it isnt monopoly like this just because someone owns the brand. Otherwise there would be tons of monopolies out here, i.e like Coca Cola has monopoly on their cola, and that Pepsi has monopoly on their cola(in one way i guess that you can define it like this though, but this isnt how "monopoly" is usually defined). Much of the stuff that are on sale on the PS Store are 3rd party stuff that can be bought other places as well.
Valve cant set the pricing in 3rd party games. I think Vale says to publishers that they are gonna have a sale, and publishers know that people buy the games then. Plus there are no distribution costs in DD, so its a win-win.
I know thats what monoploy means, but I dont know the word for this.
On PC, STEAM is not the only store that sells games as direct donwload, theres, Green man gaming, GoG, Gamersgate, Direct2Drive etc
On PS3, PSN is the only store you can use to buy DD games. Thus they dont really have any competition.