You are describing one specific implementation of the income tax. I have three arguments against your attempt at distinction; the first being that what you are proposing is essentially Nozick's 'gracious master' who takes a portion of your earnings (income tax), while retaining the ability to recall you (conscription or jury duty). Another point would be to simply point out that you are specifying 'income tax'; a poll tax with no cutoff is also possible, whereas I never limited my argument to this single implementation, and you have constructed a straw man. My third argument would be that you never took the trouble to define 'tax', and that if you did, you would see the fundamental similarity.

What you call a 'social obligation' seems to be a construct for the rationalization of abhorrent behavior, especially because the moneys collected via taxation have frequently been used to finance the oppression of a great number of people.