The block is enforced through a simple redirect using the browser's user agent. If the user agent includes 'Windows Phone' and visits the mobile version of Google Maps, the user is redirected to Google's main page - no explanation, nothing. In addition, it seems a similar block has been put in place for mobile Gmail. I distinctly recall being able to access the full mobile Gmail web client on my HD7 running Windows Phone 7.x (confirmed by Peter Bright), while both my HD7 and my 8X are now limited to the crappy basic HTML version.
The assumption was first that this was a simple bug, unintentionally caused by changes by Google. It seemed hard to believe any company would stoop as low as to use a basic user agent-based redirects to prevent users from a perfectly capable browser from accessing its services. And yet, that's exactly what Google is doing - it simply admitted to such in a statement to Gizmodo.
The thing is - it worked just fine until the redirect. Probably not as optimally as it would on Chrome for Android or Safari for iOS, but it was hardly truly bad. In fact, you can still visit Google Maps on your WP device today through some links not yet blocked or by changing the user agent, and it seems to work well enough.
Microsoft responded to Google's statement, stating that "Internet Explorer in Windows Phone 8 and Windows 8 use the same rendering engine" - Maps works fine in Windows 8's IE10. However, that's the desktop version of Google Maps, whereas this is all about the mobile version.
Google's move is understandable. Microsoft's sleazy extortion racket has forced many Android vendors to pay for Microsoft's crappy software patents, so it only makes sense for Google to retaliate in some way. The extortion scheme was already incredibly consumer-hostile, since it increases handset price, but this move by Google is probably even worse from a consumer standpoint since entire services are being blocked and/or limited.
Google has no obligation to make Maps work under the mobile Trident browsing engine - just like Microsoft has no obligation to make its stuff work under Android or Linux - but actively blocking it is an entirely different ballgame. It's pathetic, childish, and only hurts legitimate consumers who have nothing to do with multi-billion dollar corporations having a penis size contest.