Generally, employment law is not really up to date with the reality of remote work. But it typically follows the employee, not the employer. So a California company can't hire an employee in Germany unless the company is set up in Germany under German law to do so. In particular, it would have to comply with German laws regarding working conditions, as well as the relevant laws and procedures concerning income and social security taxes. California law wouldn't apply because the work is not being performed in California. As far as I know, this is the same in all US states.
Companies can generally hire contractors from other jurisdictions, however, so the California company could establish a business relationship with someone in Germany who does business as a sole proprietor. As far as I know, no US state would forbid that. But the company still has to take the foreign labor law into consideration, since most jurisdictions have certain criteria which, if not met, cause the worker to be classed as an employee rather than a contractor regardless of any claims to the contrary that may be made in the contract.