It was a Rule they broke. No weapons allowed at a sitdown. The two or more parties are supposed to talk things out. Weapons outside the meetings are fine, but can not enter when a sitdown is taking place. Castallano and Persico are to blame for it, as both told Messino to protect himself, which he interpt that he got the OK to bring the weapons by the Commission. Delocroce even was involved, he too broke that rule. The Colombo and Gambino crime families wanted this to be done fast and end the feud so they could keep making money with the Bonanno's and have an allie on the Commission. Had they waited for the three capos to exit the building with Lino then kill them, then that would have been fine.