The second is that the bulk of our LEO forces are made up of either bullies or those who couldn't pass a high school exam other than one about sports or Phys Ed. IOW, they aren't the brightest bulbs in the box and couldn't care less about it.
These people won't question if the order they are given is "lawful" or not. They will just follow it. Those who are more mentally enlightened seem to gravitate into other professions even if they start off as LEO or Milspec. So the number of limiting personnel is very small compared to those who will "just do it."
As pointed out already, the Katrina aftermath proved that point. A further point is the Cinco De Mayo "May Day" melee that happened in Los Angeles 2 yrs ago where officers "attacked" anyone including news crews, MEDICAL personnel, and infants along with the adults in the park. Note that NOT ONE SINGLE officer has been determined to be so at fault that they were terminated from the LAPD even though the LAPD made an official finding that their policies were defective and the orders that were issued were "illegal" and that there was a "breakdown in command."
Somehow I do not take comfort in the thought that our LEO and military would choose to disobey an order to attack civilians on the basis that the order was "illegal." Most empirical evidence would show that the order would be obeyed.