As to your point about cop shows being easy to sell to the public, really, what's the point of setting the show in Batman's fictional city and avoiding all comic trappings? Why not just make it a real city? Easier still to sell.
What's the point of setting some cop shows in New York, others in Boston, others in San Francisco, and so on? What's the point of giving one detective an eidetic memory, another a knack for reading faces, another a forensic-scientist consultant, another a mystery-novelist partner, etc.? The goal is to follow a formula while also making each show a distinct enough variation on that formula to stand out.
Besides, it's not about fictional settings versus real ones. It's about the fact that Nolan's Batman movies were hugely successful and profitable and so of course
they'd want to ride those coattails somehow. Heck, Arrow
is already an emulation of the Nolanverse's style and approach, and it's doing pretty well. Why wouldn't they want to promote a show as an actual part of that universe? There's money to be made there, and that's the answer to any and every question about why a television network or a movie studio makes a decision.
Using real cities is part of the charm for me. Without an Arrow who cares about a Star City? A viewer can't invest into a show. Without a Batman who cares about Gotham? It is the weakness of the Canadian cop shows which try to hide the fact they are Canadian so they can sell to American syndication as opposed to a Memphis Beat, The Glades, an In Plain Sight. The little local touches are force multipliers.
Agents of Shield uses the entire marvelverse even without people being identified as mutants to explain the supernatural which is not a problem since New York survived an alien invasion and the Hulk and Tony Stark is flying around. I would think that Gotham would have to embrace the supernatural also with others like Kick Ass having followed the example of The Batman. I can see some crime lord calling himself the Riddler or the Penguin being talked about by the detectives, anti-crime SWAT or radio patrol car cops, whichever a Gotham PD focuses on. But in the end without the super powered or just the slightly extra powered Gotham PD is just Rookie Blue.