Section 8(a)(3) of the National Labor Relations Act (NLRA) prohibits an employer from discriminating against an employee with regard to hiring, tenure, or any term or condition of employment for the purpose of encouraging or discouraging union membership. Recently, the National Labor Relations Board found that Starbucks improperly terminated a pro-union employee who got into an altercation at a store with a manager that involved the employee yelling obscenities in front of customers. The Board’s decision that Starbucks was motivated to terminate the employee because of his pro-union support was based, in part, on the following facts: the manager provoked the altercation with the employee; the manager was not disciplined even though he also used obscenities; other employees who engaged in similarly disruptive or insubordinate misconduct were treated more leniently and; the memo documenting the discharge stated that the employee was ineligible for rehire because he strongly supported the union.
Just as with discrimination based on Oregon and federal employment laws, employers need to impose discipline on employees without consideration of their union membership or support, or face potential liability under the NLRA. The NLRB decision can be found here: http://www.nlrb.gov/cases-decisions/board-decisions