I'm feeling lazy myself
First, let's use some basic logic here.
Cardio in the morning on an empty stomach would mean that glycogen and ATP are depleted.
Doing cardio while glycogen depleted = doing cardio with no immediate energy.
When there is no energy for the body to use as fuel, this is starvation, regardless of the amount of food you eat. The body literally has one purpose in life (well, i guess reproduction could be a purpose, but that's more of a species thing) and that purpose is survival.
Fat is a much more efficeint energy source for survival because it is not energy costly - ie is does not require calories to sustain. On top of this, fat is slower burning, as it is more calorie dense. Therefore muscle will be burned before fat when in a state of starvation, as fat would keep you alive longer!
This applies more to us with weight training experience because we have (or should have) "excess" muscle sitting on our bones. This excess muscle is detrimental to the bodies ability to survive. The body will burn this muscle every chance it gets. To build muscle we literally have to FORCE our bodies to do something it is not meant to do. It will not synthesize proteins that are not needed by the body (except maybe the appendix?).
Trust me, there are a host of physiological reasons that cardio in the morning on an empty stomach is very very bad, but I really don't care to get into it (again). If you really care, then I'll post it