How is it not an infringement? The sign has "the power of law" meaning that the government backs the sign and has the ability to enforce it (with their armored personnel carriers, body armor and high powered rifles).
The law is to protect the rights of the business owner. If we did not have laws to protect people's rights, and if the gov didn't enforce those laws, how would we have any rights? Well, I guess the business owner could enforce it himself, but that might get crazy.
Does a no trespassing sign in your front yard violate my right to travel?