There are 2 good reasons distilled water is reccommended to use in the CPAP.
As mention before, the scum/mineral issue, in the water, and left in the water reservoir after evaporation.
The other reason relates to bacteria. Pure water with no minerals does not support bacterial growth well, but TAP water usually has minerals in it that do contribute to bacterial growth. It is a warm moist location which promotes growth. Also most tap water does contain some bacteria. If the count is too high, it is considered unsafe, but if the count is low, it is safe for healthy citizen consumption and deemed potable. Bacteria can travel by air, and the air flow goes into your lungs.
Distilled water is cheap. The risk to using tap water is low, but the reason for the strong recommendations to use distilled water is the companies lawyers don't want liability when a problem does happen. And it will, to somebody somewhere. Don't be that person....?
Question. Did I do the study? Yes.