Except Microsoft would be getting a cut of Valve's transactions just like it is on XBox Live.
This is not true. Developers are free and encouraged to use their own transaction system for in-app purchases if they have a capable and mature transaction system already in place. If they do so, then Microsoft doesn't take any share of the revenue.
If a developer doesn't have a transaction system like this, then they can use Microsoft's in-app purchase model, for which Microsoft gets 30% of every transaction.
(That 30% also goes to 20%, for both in-app purchases and initial sales, after the developer makes $25,000 in total revenue between all such transactions)