I would not consider Serpentis "pirates" in the standard definition.
Instead, I see them as illegal corporate paramilitaries. Their focus is on booster production and feeding the narcotics market in the Federation, not pillage/plunder, or in the case of Cartel/Guristas, conquering territory by claiming tribute. Their focus is not power in this regard. Take a look at the extent of the Serpentis, just a constellation, yet they are able to give the second-largest nation a run for their money. This is contrasted with the Angels and Guristas outrightly controlling an entire region each.
I see it as the Serpentis attempting to leverage in on the Federation's business community through fronts...maybe the SDII have trouble with them...
"Agent, we need you to infiltrate this warehouse owned by GenericCorp. Obviously, we can't just go in because we'll be breaking the law, the free flow of business and all that, since we don't have any proof. We need you to hack into their secure computers and find evidence that they are actually a front for the Serpentis, supplying narcotics to this area"
This is contrasted with the more generic piratisms of the Angels/Guristas...
"Agent, the Guristas are plundering some village on a remote lowsec colony, go take care of it by eliminating them".
May be hindsight by CCP that they'd rather not call them pirates, but just rogue states.