How many times have you needed one?
Police apprehend criminals, very different from defendng.
Mass shooters and gang bangers pick their magazines and use the full capacity all the time. I've not heard of a defender needing to do that.
The Second Amendment is an individual right that is not needs based. I don't have to justify needs to anybody except myself. You have a hard time understanding that concept.
Mass shootings in this country hit a peak in the 1930s. Despite a few high profile mass shootings in the last few years they are at a very low rate compared to history.
By the way, the great majority of police never fire their weapon except for qualification during their career so if your criteria for need of a standard capacity magazine is actually having to shoot at a bad guy then most police would fail that test.
Police are not usually present when a crime takes place. They come later after the crime has been committed. It is more likely that a civilian will have the need for the firearm and the associated magazine than the police. If the police need standard capacity magazines then so do the citizens. They both have the threat of the same bad guys.
Why should I be restricted from owing standard capacity magazines because some gang bangers use them illegally? Taking the magazines away from me is not going to stop them from getting what they want.
Just mind you own business and don't worry about it. If you don't want a standard capacity magazine then don't buy one.