I doubt that it is illegal everywhere in the USA. I can buy plug kits over the counter, and that would not be possible if tire plugging was illegal. I also did a web search, and found that while no one in the industry recommends rope plugs, they are not illegal. Check this out:
http://rma.org/tire_safety/tire_maintenance_and_safety/tire_repair/
Maybe unethical could be a better term for your current employer.