Again, though, this narrative falls apart when outside of some states (looking at you Dracs) Mercs have been shown to perfectly legally purchase, salvage, and/or own transportation and combat aerospace assets, BattleMechs of all shapes and sizes, and so on. At that point, I feel that that artillery falls very well within the bounds of established precedent. Really, there are very few things that are widely withheld from non-state actors, usually WMDs and certain experimental technologies.
BattleTech may be feudal, but that doesn't mean that every bit of 9th-15th-century legislature and policy applies in equal measure.