It's because of a new law called the "South Carolina Shooting Range Protection Act of 2000." One rabid anti-gun state legislator, that is part owner in the company I work for, said that the act was necessary to make sure that in 20 years from now, there were still places for police practise and for the state DNR to hold hunting safety classes. He claimed that at the current rate of range closings, South Carolina would be without a single shooting range within 20 years. He, even though he hates guns, recognized the problem with that. He pushed the idea on his fellow democRATs by saying that it is better if people have a central, safe place to shoot (where I assume he meant where they can be watched) rather than having every one find their own, possibly unsafe, makeshift range.
It's amusing you think the grammar is strange. The state law specifies the language. It wasn't chosen by the individual range owner. The signs have to be placed on all state or federal maintained roads at a 1 mile radius from the range.