Though I'm against monopolization (anyone should be, really), there is one thing that I will defend: if Valve created something so successful and innovative, they should be able to use that to its fullest extent. Why should Valve have to limit its own success? If anything, this should push other companies to create rival digital distribution services in order to topple Steam.
Think of Steam like your neighborhood Wal-Mart. Sure, we can argue economics for ages, where one person stands on the side of the suffering small businesses and the other defends the corporate giant. See, the thing with services like Wal-Mart and Steam, is that people flock to these services because they see value in them. If a Wal-Mart springs up in a small community thriving with "Mom & Pop" stores, those small businesses must find a way to innovate their product and prove to their customers that they are still worth the extra few dollars, or the inconvenient drive down the block.
Same with digital distribution services. If Steam really upsets that many people, perhaps a competitor should come up with a digital distribution service that people can see value in. Sure, it's not a difficult task, but it certainly can be accomplished. Or better yet, why not have retailers remind us why (gasp) physical copies are still worth it?