I have little doubt that Mr. Buffett was asked... however, he has a special arrangement with the Graham family, and they would not have required (legally or otherwise) his agreement. He would have agreed, however, for the reason Sean indicates. Washington Post company has a lot of very good assets, and from a business perspective, the newspaper was seemingly holding them back.
You will also notice that, according to the 13F, that Berkshire has sold the shares in Gannett...
Berkshire is buying up some newspapers that have a close relationship with their community (something the Washington Post does not really have anymore), with a sensible Internet strategy (something the Washington Post DOES have VERY good... a big reason Mr. Buffett continued to like it), and at a very cheap price (as Sean points out, the valuation was a bit rich for the Post). As a result, Mr. Buffett would not likely want to buy it... and this deal helps enrich the remainder of the company and give the newspaper a much better chance at survival under Mr. Bezos.
PS; Sorry for the long post to answer a simple question. This says similar to what he has commented quite often over the past couple of years... the newspaper industry is in big trouble, and faces bad long term economics.