The risk of one of the Borg companies doing this to you at any time is yet another reason why you should not attempt to compete on price, and relatedly why you should probably not seek the business of equisitvely price sensitive customers.
If you send that level of email a month (which is easy to do if you send a weekly newsletter to a few hundred thousand users) you're now talking about saving yourself a few grand a year with Amazon.
Given the "switch" can be as simple as changing some SMTP settings, this is a big deal. I don't think the people who jump ship are necessarily hyper-price-sensitive customers. Amazon is just turning this service into a commodity.
You can, however, configure an MTA to deliver through SES by delegating delivery to a Perl script (e.g. http://docs.amazonwebservices.com/ses/latest/DeveloperGuide/...).