I was on the end of similar demands from one who as thankfully left AW, detailed here:
https://www.ukpunting.com/index.php?topic=284010.0You've got to question their motivation for being insistent on you parking in a specific place. It's not normal behaviour.
OK, safety and security is one thing, but anyone intent on harming an SP isn't going to fall for it, or use their own car/ reg. plate anyway. And if your personal safety is highest on your priorities, or you're risk averse, you probably shouldn't be in this game anyway, as a punter or as an SP.
So you're left with a) blackmail b) damage or c) theft from a motor vehicle.
The latter is the most likely as a) is hard to do, b) has no gain, but with c) their accomplice will know how long you'll be inside the SP's place for. Instant monetary gain with minimal risk if you've anything worth taking inside your car.
And who needs the hassle of a broken window, even if you've not.

For me it's the only thing that computes, but maybe I'm thinking too logically.