I've seen it a couple of times. Same thing happens with the 6G30 addon. There's no reason for it which I know of.
You can 'hack' the mission.sqm and remove the addon from the list right at the begining of the file. It's really easy. Open the mission.sqm to notepad and see where it says bis_resistance. Then save the mission.sqm in notepad and compile it with PboTool or UnPbo or something like that, NOT with the ofp editor because the editor will add the bis_resistance again.