I have a feeling that 37signals has tested that page extensively. It may be that they convert better when the page designed that way.

I personally like the sparseness of www.checkoutapp.com, but that doesn't mean that 37signals' page is "too cramped." The only way to know is to test.

Look at Amazon. They have been through so many iterations and have tested so much and their pages are still too cramped for my taste. It probably make good business sense for them to design them that way.