This comes right from the USGA website. http://www.usga.org/news/2012/November/Proposed-Rules-Change-to-Prohibit-Anchoring/
New Rule Would Define and Preserve the Nature of the Stroke
In proposing the new Rule, The R&A and the USGA concluded that the long-term interests of the game would be served by confirming a stroke as the swinging of the entire club at the ball. “Throughout the 600-year history of golf, the essence of playing the game has been to grip the club with the hands and swing it freely at the ball,” said USGA Executive Director Mike Davis. “The player’s challenge is to control the movement of the entire club in striking the ball, and anchoring the club alters the nature of that challenge. Our conclusion is that the Rules of Golf should be amended to preserve the traditional character of the golf swing by eliminating the growing practice of anchoring the club.”
New Rule Would Address Recent Developments in the Game
This proposal reflects The R&A’s and USGA’s responsibility to define how the game is to be played. Aspects of how a player must make a stroke have been addressed in past Rules changes, such as the century-old Rule codifying that the ball must be fairly struck and not be pushed, scraped or spooned and the 1968 prohibition on the “croquet” style of putting. “As governing bodies, we monitor and evaluate playing practices and developments in golf, with our primary mandate being to ensure that the Rules of Golf continue to preserve the fundamental characteristics of the game,” added Davis.
Although anchoring the club is not new, until recently it was uncommon and typically seen as a method of last resort by a small number of players. In the last two years, however, more and more players have adopted the anchored stroke. Golf’s governing bodies have observed this upsurge at all levels of the game and noted that more coaches and players are advocating this method. The decision to act now is based on a strong desire to reverse this trend and to preserve the traditional golf stroke.
“Anchored strokes have become the preferred option for a growing number of players and this has caused us to review these strokes and their impact on the game,” said Dawson. “Our concern is that anchored strokes threaten to supplant traditional putting strokes which are integral to the longstanding character of the sport.”
What it basically gives is the refined definition of what a golf stroke is by the USGA and the rationale that it was an acceptable stroke until too many people started using it.
How can those against the ban make an argument when the USGA's rationale is basically the stroke was okay until too many people started using it. For those that used the parent analogy it's the same as a parent telling their kid, "I was okay with you going to your friends house when you only asked once or twice a month but now that you're asking a few times a week I no longer deem it acceptable for you to ever go to your friends house."
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Originally Posted by
jamo 
To your first point, "we don't like belly putters nah nah nah nah boo boo" was not the USGA's reasoning. Read their press releases, or anything they've said since the ruling went public. To their credit, some people have made real points, like Stretch, but the vast majority of objections I've heard are misrepresentations, emotional whining, or illogical.
To your second point, people have been asking for the USGA to look at this for years.
