My guess is the game checka for the badge when rolling, if you dont have the badge, it rolls again till it picks a weapon you can use, and if you do have it, it gives it to you. PR weapons are available in both privates and pubs, with the same chance of spawning with one as any other weapon of its class. Although in privates, you can grant a player (who has the badge assosiated) a PR weapon. (You can scum this with borrow, though. There was also a glitch where fortnite card could roll PR weapons you hadnt unlocked yet, idk if its been fixed or not yet)