I'd also add that there's a socioeconomic component. In Missouri, at least up until 2025, you'd get your temp tags when you buy the car, and your actual metal plates once you paid sales and property tax and registered the vehicle with the DMV. This recently changed to make the sales and property tax apply at the time of the purchase so that you'd get your plates much more quickly after.

A car is a necessity in most of Missouri. Kansas City has more highway miles per capita than any other major city in the country (and maybe in the world); IIRC St. Louis is fourth-most highway miles per capita. Public transit has major gaps. Inability to drive is such an encumbrance that those convicted of DUI are allowed to petition courts for a hardship license allowing them to drive to work and other essential places because not allowing for this could fail under the Eighth Amendment.

All of this is to say that if you are able to pay for a car, but not the sales tax for the car, and you get pulled over for not registering after your temp tags expire, you are essentially under house arrest until you can put together the money to both pay the fine and to pay the tax on the car, which is now exponentially harder since you can't drive anywhere. Since that'd put disadvantaged people at an even greater disadvantage, it might be a "community relations" move by the PD to look the other way on these cases, at least until another blatant violation occurs.