It's pretty much to make sure people who have moved into the city aren't skipping out on paying for a residential parking permit.
When I first moved to DC it took forever to get a residential parking permit because to get it, you need your car registered in the district, which needs a dc driver's license, which needs proof of residency, which if one doesn't have a lease and you aren't on any of the utility bills, is a little tough. For those two months I was at work in Maryland most of the day, and so could legally parking all night starting at 6:01pm. Eventually they gave me a ticket for always being parked in the neighborhood but not having any DC plates or parking permit.
I could see how frequenting the 9:30 club a lot (and hence having one's car in the area often at night) might make parking enforcement think you're a registration dodger.