I would assume that the AWS guys are all about comparative advantage. Anybody not familiar with the term will probably enjoy Wikipedia's surprisingly good explanation:
tl;dr: The AWS people get higher marginal return on their investment of time and money by hosting with TypePad and putting the time they save into making AWS better. Probably.
http://en.wikipedia.org/wiki/Comparative_advantage
tl;dr: The AWS people get higher marginal return on their investment of time and money by hosting with TypePad and putting the time they save into making AWS better. Probably.