Magento is enterprise class software. It runs on EC2's hosted at Amazon Web Services - which is as prime heavy duty cloud hosting as you can get.
I suspect their store database instanceType is far too small and wasn't ready for the newsletter onslaught (slashdotting effect) that it got
Bit of better capacity planning between marketing and Amazon team would have sorted this right out! They could have scaled the RDS instance up for the burst of activity that the mailshot was to generate, then just scaled back down later on.