Some of the worst offenders are the "liquidators". When a store (usually funiture stores, it seems) goes out of business, they often hire a "liquidator" to do it. The liquidator essentially buys the remaining inventory and then holds an "inventory clearance" sale.
Although the state has specific laws pertaining to store closings, partly to prevent the liquidator from just hauling in new junk and continually restocking, it is not really enforced.
These are outside, national companies that do not care about the local area or what they do to it. They just want to make a quick buck and disappear, so they put up a glut of signs, on- and off-premise, regardless of the local law. Even if Code Enforcement gets involved and issues correction notices or citations, the sale could be over and the liquidator gone before the day of reckoning in a hearing. Thus, the only defense is for citizens to constantly remove the signs.