That's something that needs to be brought up when they are dealing with the amount of support to be paid. The OP (and some of the posters here) seem to be missing that. Assuming he was always an IC, then CS was set based on the information the court had at the time. Whether or not he's already paid taxes on it (as he stated) is irrelevant, most people pay their support via wage assignment before they pay taxes on their paycheck.
I think I touched on that topic earlier when I mentioned that possibly OP's problem stemmed from not cooperating with the CS process.
However, I am going to disagree with the bolded part. Under federal law, garnishments have a maximum set percentage of the paycheck AFTER TAX. In other words, the net rather than the gross paycheck. I know this from having been an employer/CFO for many years. Therefore there is some possibility of unfairness in garnishing IC income, even if the child support was calculated fairly.
In fact, if I were someone subcontracting work, and being required to garnish IC income, I would be nervous about violating federal law regarding garnishments, in obeying CS garnishments.
A true IC payor, has no idea what their subcontractor's net income is. They have no idea what their subcontractor's payroll expenses and business expenses are. Therefore they could easily violate federal law by garnishing more than federal law allows.
Of course, if they are improperly treating their employees as subcontractors, which is all too common these days, and all too ILLEGAL then they wouldn't have to worry about violating federal garnishment law...sigh...it would be easy to figure out.
As an accountant, and as a tax professional, and as someone who has been an employer or has been responsible for following federal law as CFO for a company, I am extremely uneasy about the idea of garnishing IC income.
As a taxpayer however, I am glad to see that IC income is also being garnished for CS.
However, it is a real catch 22.