.... Well ..... If we weren't under the thumb of an oppressive administration .... normally this wouldn't be of concern.
.... That said though ..... These types of operations are probably staged in an effort to teach all involved about learning to effective traverse terrain and operate equipment in this environment. However ..... My guess (and i am not an expert and don't profess to be one) is that the main reason is that there is also a serious need for law enforcement to also learn the different authorities and command structure during a joint operation/mission with the DoD ...... otherwise there would be mass confusion about who is in charge of what and the different or shared responsibilities that would be delegated to and handled by whom etc etc. Heck ..... It is confusing enough when a basic local crime scene is being handled by the local authorities .... and the Feds show up and claim jurisdiction. This kind of stuff actually does need to be addressed.
Posse comitatus comes to mind based on your response...
It generally prohibits Federal military personnel and units of the United States National Guard under Federal authority from acting in a law enforcement capacity within the United States, except where expressly authorized by the Constitution or Congress. The Posse Comitatus Act and the Insurrection Act substantially limit the powers of the Federal government to use the military for law enforcement.
IMO, this is just one of many attempted end runs around the PC Act.
Sheriff and PD Depts. have plenty of prior military personnel who can instruct the Depts. on various tactics learned while in the military...