The reason is that open source has bigger enemies than Microsoft when it comes to VOIP technology. The Bells.
While Verizon has lately been trying to kill VOIP with price cuts, a quick read of the fine print on its VoiceWing service shows just how much deviltry it and AT&T can engage in regarding VOIP. The whole network neutrality debate has been, in large part, about Verizon and AT&T seeking to control what software, and whose software, you use on connections you buy from them.
When AT&T or Verizon attack Skype or Vonage or even Digium in this way, it's going to be an uphill fight for customers to get redress. Do it to Microsoft, or a Microsoft customer, and you're in a different situation.
Even though this is an OEM product, and Microsoft's name won't be on the boxes businesses buy, Microsoft is the ultimate vendor, and will doubtless stand behind its product's legal standing.
With Response Point Microsoft has put its lawyers at the disposal of supporting the technology Vonage and Digium pioneered. They are competitors in the market, but in a law court they will sit on the same side of the table.
I like having good lawyers on my side of the table, especially when someone else is paying the bill. Don't you?