Kill the humans
Why won't they give discretion to the parking inspectors? If they catch the substitution, it's far cheaper than issuing a citation that may be challenged further up the line. The further it goes, the more cost is added.
When I had a manufacturing company, we had inspection steps all along the processes to catch mistakes and bad components. If we only did one final QC test at the end of the line, we might have missed things that should have been rejected straight away. I see the citation issue as being very analogous. Unless, of course, the idea is to assess lots of fines where many people won't take the time to fight them.