Two points;
1> Because the Constitution says that income taxes are fine, and you knew that this was the case when you made that agreement between yourself and that private group to exchange money for labor, knowing that a portion of that money would be owed in income tax.
And;
2> There's no 'double taxation'. Money isn't taxed, transactions are taxed. You being paid your salary is a transaction, taxed. You paying for anything else after that is an additional transaction, and also taxed. Just like how whatever the person you bought from uses that money for, that's another transaction, taxed. The concept of 'double taxation' is completely nonsensical here; it's applicable in a very particular set of circumstances, with regards to income tax and corporate tax, and even there it's squirrely, but if you're trying to apply it to income and sales taxes, you're just demonstrating a complete failure to grasp the basic nature of taxation in the first place.
You have literally zero grounds on which to stake this claim. Particularly since, as above, you knew about that income tax when you arranged your salary. It's like arguing that your rent is "unfair" because you worked damn hard for that money. It's a silly argument.