There are ways to prevent DMA over firewire, and also Apple has been aware of and provided prevention methods (such as disabling firewire DMA when a firmware password is enabled) over the years.

A good account of the current state of affairs is here:

http://derflounder.wordpress.com/2012/02/05/protecting-yours...

And a diagram of protection methods that do work:

http://www.frameloss.org/2011/09/18/firewire-attacks-against...