I think probably it's too early to expect the big CDNs to start doing this. When enough sites are using a library, the benefits become clear: greater availability, more frequent use, reduced client requests, increased performance for the end user. If only a few sites are using it, chances of client having a copy in their cache already is low and all performance boosts are lost. So all that's left is that MS and Google offset your bandwidth charges, which is not their intention. Thus, the solution is to get more developers to use the library.
Plus the library is so tiny. The code is still only 3.5KB using conservative minification. For comparison, jQuery is 24KB and ext-core is 29KB. I'd personally recommend folding the library into your own site's base JS and get your performance boost there. At least until there's wider acceptance.
Plus, it's funny I'd have expected the JSON library to be hosted also at Yahoo, but I can't find it. I mean Crockford works there.