Parking tickets in the cities which have these sorts of sign issues (DC, NYC, Chicago, ATL, etc) make up a significant amount of income for the city. Paid parking certainly adds up but if a single space nets the city $10-$20 per day, a single parking ticket could cover a weeks worth of revenue for that single spot while still allowing the space to earn $10-$20 a day for the rest of the week.
I agree it sucks, and I was burned by this exact problem when I traveled to DC with my family last winter. However it's unlikely to change, because not only would the change actually cost money to implement, it would (significantly) reduce the income to the city from parking tickets.