Barely braking a sweat when you do cardio = good...

Ok to sum it up, i always see pros like Cutler etc, walking or barely braking a swet when there doing cardio, there never actually pushing it that much and heres my question, simply why?
 
I thaught about it and figured if they do cardio straight after weight sessions to really burn some serious fat, do they simply draw it out for 20/30 minutes none stop with low intesity because its the best way?
 
PimpWivGunZ

Simple answer yes.

After weights, is the best time to cardio, also at low intensity as it burns more fat.