I really don't think Tesla has really grasped the difference in relative power between US and Nordic unions, or European unions more broadly. E.g. the unionization rate in the Nordics ranges from around 50% in Norway to around 70% in Denmark (numbers vary, these are conservative numbers).

Even in Norway, with the lower union membership a large part of the reason for that is that the union power is sufficient that a far larger proportion of the labour market is covered by collective bargaining agreements.

For Tesla to then just outright reject collective bargaining is then seen by many as a direct attack on what has been the norm for decades, and on organizations representing most of the working population and holding a lot of real power and with the ear of politicians. Expect that if Tesla somehow gets the unions to yield, it'll be followed by subsequent election campaigns being fought in part on changing laws to reign them back in.