(CBS News) — The U.S. Supreme Court has dealt a blow to public sector unions, holding that requiring workers who are represented by a union, but choose not to join, do not have to pay dues to cover the costs of collective bargaining.

A narrowly divided court ruled that such dues, often called “fair-share fees,” violate free-speech rights of union non-members.

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Such fees violate “the free speech rights of nonmembers by compelling them to subsidize private speech on matters of substantial public concern,” the majority wrote in its decisions.

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Most legal observers had broadly expected a loss for unions.

The 5-4 decision by the nine-member court was widely expected to devastate union funding. While workers can choose whether or not to join a union, unions are legally obligated to bargain for everyone in a particular working unit.

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Across the border from Illinois, AFSCME Iowa Council 61 enjoys an overwhelming 83 percent support among covered workers — but only 29 percent of those workers are dues-paying members.