Employees have the right of self-organization and the right to form, join, or assist labor organizations, to bargain collectively through representatives of their own choosing under this subchapter, and to engage in lawful, concerted activities for the purpose of collective bargaining or other mutual aid or protection. Employees also have the right to refrain from any or all of such activities. A general employee has the right to refrain from paying dues while remaining a member of a collective bargaining unit.