Their backend software is probably able to deal with it, but not a massive DDOS brute force attack to try to find holes in the exchanges. This is an information gather exercise by some organized hackers.
Some servers used to have a resettable "case has been opened" flag in the BIOS - the pieces that was based on could be leveraged against the DMA attacks. Overwrite certain items in memory, or maybe just power off the system, when the box is opened, and obstruct opening of the box (a lot of glue? or somesuch) to extend the time past the "recover memory" window.
Yes, I realize this would still be susceptible to coercion of the humans involved, and other issues, but it could be a building block of some degree of NSA-proofing.
Yes, very easy. In a mac you just plug in a Firewire/thunderbolt device. More extreme measures involve freezing the RAM. Both require physical access to the machine, but a bit more scary that plugging your laptop into a public display/TV gives an attacker control of your computer and passwords.
Full disk encryption TrueCrypt/BitLocker/FileVault can act as countermeasures and modern versions of OSX don't allow DMA from the login screen anymore.
pyCon clearly set their expectations for their guests during this event. I think if the "community" has such a large issue with it, they can stop attending conferences that don't adhere to their social expectations. (pyCon would not get any smaller though)