Remember, U.S. income taxes are voluntary. So, when does a person volunteer? When he fills out a W-4? When he fills out a 1040? When he responds to an IRS letter incorrectly? All of the above and many more?
When you convert goods or services into USD you enter into a contractual agreement. Under some circumstances even denominating your goods or services into USD make you contractually liable. The IRS is a private corporation, and they rule over everyone via maritime UCC contract law. 