(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.

Most legal observers had broadly expected a loss for unions.

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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.

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.