FAR 91.144 talks about restricting flight operations during abnormally high barometric pressure conditions. Why would there not be an identical restriction during abnormally low barometric pressure conditions?
This seems to be the more dangerous situation. In the "abnormally high" situation, if the altimeter can only be set to 31.00 and below, then it will tell you you are lower than you are, which gives a safe bit of padding. In the "abnormally low" situation, if the altimeter setting drops below 28.00 and the altimeter can't be set any lower, then the pilot has no padding and is actually lower to sea level than the altimeter says he is.