Idaho Code § 44-901

It shall be unlawful for any person, firm or corporation to make or enter into any agreement, either oral or in writing, by the terms of which any employee of such person, firm or corporation, or any person about to enter the employ of such person, firm or corporation, as a condition for continuing or obtaining such employment, shall promise or agree not to become or continue a member of a labor organization. Any person or persons or corporation violating the provisions of this section shall be guilty of a misdemeanor, and upon conviction thereof shall be fined in a sum not less than fifty dollars ($50.00) nor more than $300, or be imprisoned in the county jail for not more than six (6) months, or shall be punished by both such fine and imprisonment.