There's a big lack of understanding flying around here.
The problem arises because the Altimeter is calibrated to ISA. Any deviation from ISA means the altimeter is going to read something other than the true altitude. This is 'temperature error' in the long list of the errors of the altimeter.
If the temp is above ISA, the air is less dense and the altimeter will under-read. In otherwords you will be higher than you think and this is safe. If the temp is below ISA, the air is more dense, the altimeter will over-read and you will be lower than you think. When flying on an instrument approach to a minima carefully calculated relevant to a nearby obstace, this quite clearly isn't safe. Some one has decided that ISA-15 is a good temp to start taking this into account.
The approach minima is referenced to the touchdown point. Whether you start from a sea level reference (QNH) or from the altitude of the touchdown point (QFE) or even with reference to the touchdown altitude with 1013.2mb set (QNE) it's still the gap between you and the ground that's relevant.