Unlikely as the terms are not clearly defined. Are the parking spots their property or council property or state property?
Are you sure it is their sign and not a council sign? If it is a council sign they may have an agreement with the council to patrol those parking spots and then you may be hit with a parking fine.
If they say customer parking only and they own/lease the parking spots why would you want to make use of someone's valuable space? They are simply trying to make a living like everyone else and it can hurt their business if their customers cannot get parks close to them. The business owners obviously feel the need to erect signs because too many people are abusing their parking.