What Tor does isn't technically illegal. Just because Tor is frequently used for illegal activity doesn't make the creators of Tor culpable for the illegal activity that happens on it. You wouldn't arrest a Toyota executive if it was found out that Toyotas were the most popular vehicle for drug smuggling, would you. Not unless the executive was specifically marketing to or enabling drug dealers.

The United States could probably introduce legislation that would make Tor illegal and then spend the decades working with other countries to try and get it shut down. However by the time they got any of that legislation Tor would be changed just enough to be legal or would be replaced by a copycat that did follow the new legal guidelines.