I struggle to grasp the logic supporting this conclusion. Perhaps it's a definitional miscommunication. I suppose it's possible that someone could think that private ownership of the means of production is the same thing as cold-blooded homicide, but I think it's more likely that the author of the sentiment confuses capitalism with government-backed crony corporatism. True, it doesn't have the same ring to it, but the case that Blackwater is a band of thugs is more defensible than, say, Johnson and Johnson being bloodthirsty mercenaries.
"Know thy enemy" springs to mind. I suspect that adolescent anarchists (anarcho-communists, more probably, based on some of the other graffiti in the area) don't know a plugged thing about how the relative ease of transfer of goods and services have contributed to the post-magical abundance each and every one of us enjoys today.
Interesting status dynamics arise in these sorts of subcultures. There's a funny balance between retaining outsider status, which can confer high status within the group (though, low status, obviously, outside) and attracting new adherents. As the numbers in the group swell, the local high status position is threatened. If the original members are unable or unwilling to adjust their status expectations, they will splinter. It becomes a matter of how highly they value outsider status compared to how much they value their cause. I suspect that members of splinter factions more often cherish being misfits. The label of "sellout" is extremely low status, to be avoided at great cost.
Fight the power!