It's all about distributing force. The more area the motor mount is distributed over, the less likely it will be to damage the surrounding foam. You can increase your motors surface mount area using thin plywood, or a material stronger than the foam you use.
It's hard to see, but here is my delta. The motor is mounted to the end of a simple stick mount, and that is attached to a 2inx4in 1/8in piece of plywood glued to the main foam airframe. Very unlikely to come off in a crash.