I think that some of them are probably sold to the Jennerit before Rendain sided with the Varelsi, some of them are sold by groups within the LLC that don’t care about the end of the universe as much as they care about profit, and some are quite possibly stolen. They only appear in Rendain’s service in the prologue and the Experiment, which (to me) implies that the Jennerit may be running low.
On the other hand, another possibility is that they are all malfunctioning LLC property. The only times that we see enemy minions are on Tempest (where Varelsi portals are doubtless getting opened all the time), and during the Experiment (which involved manipulation of Varelsi portals). Kleese does mention during the Edge of the Void that Varelsi portals “have a dizzying effect on A.I.”, so it isn’t too great a leap to assume that these enemy minions are malfunctioning to the point that they are willing to attack any enemy EXCEPT those that they were programmed to attack (Varelsi and Jennerit Thralls).