It's not just that they don't require employees to travel there, they also don't pay for trips "This bill would prohibit a state agency from ... approving a request for state-funded or state-sponsored travel to ..."

The bill doesn't explicitly forbid government employees (and by extension, the government of California) from doing business with North Carolina - it just adds personal, out-of-pocket cost for doing so. (which probably amounts to the same effect.)