The cameras that they have to read plates in a lot of different conditions and various states of cleanliness. Some states allow O and some states allow 0, and some states don't care. Combine the two issues and cops get lazy and want to check the plate with both the 0 and O just to "make sure".

The cameras also confuse D and Q with 0 and O. And 5 & S, and 2 & Z, and 6 & G, and 8 & B.

The cops have likely been doing this for decades, because the human eyeball can confuse an O for a 0 much worse than image recognition does these days.