I see your logic and had the same thought, but then wouldn't a "No stopping at anytime"/"No parking" sign accomplish the same thing? "No Loitering" sign as well. If the point is for the cop to be able to point to a sign, those already exist and are much more broad and easy to use(or misuse, depending on your perspective). Like others said, an absolute minimum speed limit seems destined to harm pedestrians. Plus, you can see it's before a stoplight. So not even logical.

I'm betting this is a fake sign.