They key phrase you are choosing to ignore:
“if the persons described are authorized by a competent authority to so carry an assault weapon loaded on a public way and such person is acting within the scope of his duties or training...”
That big “if” excludes most government employees and further is limited, even with respect to those exempted, to only when “acting within the scope” of their duties. It is, in essence admitting it - the assault weapons ban - does not apply to law enforcement types when such types of persons are acting within their official duties.
Gubmint goons “exempt”?.........
Looks like they are planning ahead, knowing when/if total gun ownership is NO MORE, Civil War III will begin. They want to be able to protect themselves from “We, The People”.