There’s little question that employers have the legal right to impose vaccine requirements, and this seems to be true even where working conditions are subject to collective bargaining. However what remains to be litigated is what sorts of accommodations are in fact required for religious or medical exemptions. The Biden administration’s proposed OSHA rule is expected to include regular testing as an alternative to vaccination.
United does not allow such an alternative. At 97% vaccination, United faces less operational risk than other airlines – who might see pilots take retirement if they object.