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Assuming the independent contractor is properly classified as such and should not instead be an employee, there is nothing unlawful about the requirement you describe. This is because independent contractors are essentially viewed as their own business. The law does not regulate the terms or conditions of business contracts between separate businesses--the parties are free to agree to whatever is mutually acceptable, or to contract with others instead.
Now, if you were an employee, or misclassified as a contractor when you should be an employee, a "mandatory contribution" to anything MIGHT be regarded as an unauthorized deductions
from earned wages
. But that argument is a stretch, and especially so if the deduction does not take the employee below minimum wage
In short, as a properly classified independent contractor, there is no legal issue with what you describe, and even as an employee, there probably isn't either.
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