According to their ordinances - http://library.municode.com/index.aspx?clientId=14101 - Do a search for 18-50
A building permit is required for "The installation, alteration or repair of any sign". Technically, you need a permit even to replace a light bulb in a sign. The fact that the sign company has no idea of this means it's not typically enforced. The code enforcement guy is playing hardball with this guy because he missed a protection payment or something.
The sign ordinance:
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