The problem is that FreeBSD developers are still fighting wars of the 90's. By and large, with the welcome exception of Colin Percival (of http://www.tarsnap.com/ fame), they have not embraced virtualization. The only saving grace of FreeBSD is that it has adopted Sun's ZFS. This won't hold the fort for long though.
For BSD-on-EC2 see http://www.daemonology.net/freebsd-on-ec2/