It’s not a maximum - but it’s a guidepost. It’s a less arbitrary way of defining what is reasonable. Police arguably go through analysis to determine what the most effective carry tools are for them. Piggybacking off of this analysis is a means to establish a baseline. If it is reasonable for police to carry 17 round mags in their service weapons, then it is reasonable for individuals to carry 17 round mags. This isn’t to say larger magazines are unreasonable.
Another facet of this line of reasoning is that laws that prohibit people from various things related to arms sometimes exempt police, off duty police, or even retired police. This makes no sense. Just like the proposal in the OP - when exempting hunters it’s an effort to divide gun rights advocates. The idea is that gun control folks aren’t after hunters, so they should acquiesce to various restrictions. It has had limited success in the past but I think many folks have wised up to this strategy of division.
And by including police within the the same group of people that any limits will be imposed upon, then there is the expectation that police will not support restrictions that make them less effective. It adopts every argument that a police force may employ to support their choice in arms. The military aspect arises because of the militia component and that is more of a constitutional argument.