Because microsoft is paying verizon millions of dollars (plus a cut of ad revenue) to do so, and the deal required a certain number (though unspecified) of "High End" Devices.
I'm pretty sure the deal went through sometime before the "Droid" brand was born, because Verizon replaced Google with Bing on all their blackberries as well. (via a forced OTA update). I don't think they really predicted the backlash from it. Even normal customers are getting pissed because it breaks stuff that they see others doing (Voice search doesn't work like it should, you can't open Google maps with a voice prompt, etc) So hopefully they'll be smart and not renew the deal, or at least make sure high end android devices (ideally all android devices) won't have Bing forced as the default.
As for Microsoft, they're making millions off of Android because they twisted HTC into licensing their patents and they're going after other's as well. According to Barnes and Noble (The only company that refused to sign an NDA before they heard the charges against them) Microsoft is demanding more to license their patents for Android than they are requiring for WindowsPhone 7 licenses.
Honestly, I'm not sure how Google allowed the Bing thing to happen. Sure, Android is Open, but the market is not. And if this skyhook thing has shown us anything, it's that Google is pretty involved in the device approval process. The question we should be asking is why do Bing devices have market access?