"If ethics are a matter of only subjective belief," the objectivist asks, "then how do you justify your condemnation and outright oppression of murder, rape, theft, and mopery on the high seas? If all subjective opinions have equal (i.e. equally zero) objective moral weight, then why shouldn't you simply permit everything?"
There are two enthymemes in this position, one accepted and one rejected. The accepted enthymeme is "Every choice must follow from an asymmetry in the justification of the options." The rejected enthymeme is that "Every choice must have an objective justification."
The second enthymeme is false-to-fact. We make choices all the time on purely subjective criteria. I choose to buy vanilla ice cream rather than chocolate for no other reason that I subjectively like vanilla and dislike chocolate. There is no objective reason—there is nothing at all objectively superior about vanilla—only facts about what I like. I work as a computer programmer because I like computer programming; I'm not stupid, I can tie my own shoes without assistance, and there are many other professions I could have chosen, some which would have made more money, some which would have done more "good for humanity". But I like computer programming. I married the woman I did because I like her. I live in California because I like it here. These are not trivial or unimportant decisions. The idea that one must have objective reasons to make choices seems pretty obviously false-to-fact.
Since we do act on pure subjective preference, the burden of proof is on the objectivist to show that specifically coercive action must follow from specifically objective reasons; such a premise cannot simply be assumed, much less assumed implicitly.
The first enthymeme is accepted, but easily satisfied. All purely subjective properties are objectively symmetric. However, there is a fundamental asymmetry: My subjective properties are privileged to me because they are mine (or I am my purely subjective properties). I act according to what I approve and disapprove of: I disapprove of theft, therefore I physically force (or sanction physically forcing) murderers and thieves into locked rooms where they cannot continue to kill or steal. It's really as simple as that.
Well... not quite so simple. It seems there are a few hundred million people in my own country, and six or seven billion in the world, with whom I interact on a more or less continuous basis. And—however they construct or express their beliefs—they have their own subjective beliefs about what they approve or disapprove of. Because I approve or disapprove of something is a necessary reason to act, but it's not a sufficient reason. Because of all of those people and their opinions—and their ability to coerce me—I have to consider how they feel about things.
For example, I see absolutely nothing at all wrong with smoking marijuana. I also enjoy making money. It therefore follows that I'd love to sell the stuff. However, my fellow citizens strongly disapprove, and they disapprove strongly enough that they'll put me in prison if they catch me selling weed. Since I dislike prison vastly more than I like money, I don't sell weed.
Another example: I very strongly disapprove of the United States' war in Iraq. The vast majority of my fellow citizens strongly disapprove. However, the war has been sanctioned by our political, legal process, And I approve of that process more than I disapprove of the war. (It's no small reason that I approve of the process because it does have a nonviolent methods to change such decisions, and I disapprove of violence.) I don't, therefore, take personal coercive action to end the war.
Yet another example: I violently disapprove of chattel slavery. If I were magically transported to the American South in 1840, I'm pretty confident that I would be a station on the underground railroad, and prison be damned: I disapprove of slavery more than I dislike prison.
"But what if everyone approved of slavery? Wouldn't it still be wrong?" I can only note that if everyone approved of slavery, it simply wouldn't be a topic of ethical discourse. I have better things to do with my life than disapprove of distant alien civilizations or imaginary possible worlds. I live in this world, with these subjective beliefs.