I'm started to see this more nowadays. Stores and restaurants closing and leaving up their signs. This can cause confusion, leading someone to think the place is still open.
...and their websites, facebooks, etc... Yeah, it's annoying.
I guess the sign ordinance could be amended, but it'd be tough to do that in a legal/constitutionally permissible way.
Its up to the landlord to remove signage.
Many leave them up to keep the break ins to a minimum.
For strip malls, many leave them up to make them look occupied.
Its also a cost factor. Rather than have the sign company come out twice, leaving it up saves dough.
Also, some tenants pay off the remaining lease early when they leave, so the landlord is not legally inclined to remove anything until the lease contract formally ends.
Great reasons! Now we know! Thanks Spu! 8)