The BMW head guards have rubber inserts that press against the head cover - this traps water (salt) between the rubber and the head - if not removed and cleaned regularly and liberally dosed with ACF50 or similar then the head paint bubbles and corrodes. This is likely to be the case with other brands of head protectors too. I stopped using them because taking them on and off each time I cleaned the bike was too much of a faff, so I just rely on the standard engine guards these days.