Indeed. I have been long been sympathetic to your point that the moral argument for AC isn't very compelling. Some people do reject the notion that "taxation is theft"; I think Arfinn in a previous thread convincingly pointed out that one's moral stance, which I think can only be only internally subjective, will cause the rejection of many the arguments repeated by some proponents on this board.
I think a better way to examine the moral structure of AC instead of viewing it through the veil of "forced expropriation" or "jack-booted thuggery" is to simplify your moral critique by just thinking through the concepts of negative and positive rights.
Simple thought experiments and discussions with others guided me towards minarchism, if not outright AC. Positive rights require obligations of others - and thus are inherently "asymmetric" and inequal. Negative rights, as they require none of these obligations, are ideally suited to each individual's preferences and are equal. In the realistic setting, enforcing positive rights usually entails arbitrary (actually, political) decisions about the nature and scope of those rights. Thinking about the "right to healthcare" or "the right to minimum wage" in a simple model illustrates how coercive positive rights become at the margins.
Arrow's Impossibility Theorem also mathematically proves it is impossible to construct a political system where each individual's preferences are obeyed. And presumably from those preferences his moral system and the methods he will choose to engage in business and pleasure stem.
The reality is that socialists and conservatives feel marginalized in a free system. Their ideologies are based on those very positive rights: the right for you not to be poorer than I, the right for "us" to preserve the traditional status quo. Morality to them is not subjective, but objective, and most people do not recognize others' moral systems. Insofar as their morality supersedes strict utility-maximization, they will reject the libertarian or anarcho-capitalist position.
Cliff Notes: Your point is well taken in arguing more in the utilitarian rather than the moral vein for AC. However, most will never be convinced by this because they view their morality as objective, requiring positive rights, and thus reduce their potential utility (presumably less so than everybody else). It's probably better to use your approach with the more educated however, since they have more nuance in their worldview.