Social Anarchists, Free-Market Leftists, Liberaltarians, or whatever you'd like to call them are well-known within the larger libertarian community for the animus and contention they hold for their more right-leaning libertarian brethren - whom they've often dubbed "vulgar" libertarians. I've seen the term used in two distinctly different contexts. The first, and most often used, condition refers to a libertarian who leans in the direction of being critical of lower-class recipients of government entitlements but whom often disregard the artificial entitlements bestowed upon corporations and businesses in general. To be fair, I think they make a very good point. And few libertarians who don't already find themselves in the orbit of ethics a la Rothbard often defend entities that, if they were consistent, they shouldn't.
The other context the "vulgar" prefix is used in is regarding liberty-minded people who fall under the "thin-libertarian" category. These are people who, ideologically perhaps, value liberty as the overwhelmingly primary moral litmus test. "Thick-libertarians" find that the value of liberty is necessarily bound with other social values (compassion, equality, etc.). Left-libertarians are generally of the latter variety. Many of them support liberty only so far as it is the most effective path to realize these other values. So, a thin-libertarian may believe the minimum wage is wrong because it coerces employers and restricts their freedom of association. A thick-libertarian may believe the minimum wage is wrong because it ultimately unemploys people. Not all consequentialist libertarians consider their valuation of liberty to be thickly bound but it is a point worth debating.
What strikes me negatively about many left-leaning libertarians is that, because of their thickly bound values, they often put themselves in awkward and somewhat inconsistent positions. In the article above, it seems to me that the author, although recognizing that public monopolization of various services is wrong (note: even for the consequential reasons entailed by the multiple values they hold), twists himself into the odd knot of defending public unions. Why? Well, it seems, from the author's point of view, such a defense is warranted when there are more egregious acts taking place that benefit the likes of another interest group - the owners of capital (read: business).
Now, while I think it's worth debating which interest groups are the worst offenders of what we would commonly describe as freedom, it's not clear to me how one group, even if we somehow would prove them to be less offensive than another, should somehow be defended in light of that relative status. In fact, that kind of twisted logic seems to be the logical inverse of what their right-leaning opponents so often exhibit. Right-leaning libertarians, looking at net government transfer payments overwhelmingly leaning in the direction of the lower-class, often ignore some of the more complicated benefits and power bestowed on business and end up defending these companies from the onslaught of the left in general - pointing out how (in their mind) government clearly favors labor over capital. Left-leaning libertarians, conversely, seem to be overlooking the parasitic nature of the interest groups they are defending while drawing focus to corporate welfare instead.
And, again, to be fair, not all people in the left-libertarian camp are guilty of this. But it seems to me, by their own account of what logically constitutes a "vulgar" libertarian, many of them are simply just as guilty as the people they label. This isn't confined to the current national discussion regarding public sector unions and their fate: I've seen left-libertarians defending what they ought not, by their own ideological account, simply because their knee-jerk reaction, like those they bemoan, is to provide a relative defense for a particular interest group with whom they focus on or sympathize. It appears to me that all libertarians would find it silly to deflect the wrongness of someone being arrested for smoking marijuana simply because there are even more aggressive actions taking place. Why would libertarians (of either the left or right variety) want to start behaving as such issues like this then?