Even worse, they might request your dismissal if you don't join. Here is a direct quote from the Union agreement of a major university, where I teach part-time: "The Union may request that a Part-time Faculty Member who fails to join the Union, maintain Union membership, pay an agency fee, or make a charitable contribution in lieu of an agency fee shall be dismissed. If the Union makes such a request, the Employer shall comply... If the Part-time Faculty Member fails to pay within that time period, and the Union so verifies, the Part-time Faculty Member’s employment will be terminated at the Union’s request".

If anything, unions are only acting in their own interest.