They have an anti-calcification function designed into them. The reason for distilled water was because well water can be "hard" and cause calcium deposits to build up in the iron -- especially in/around the steam vents. This increased the likelihood of it "spitting" water while ironing & caused uneven steam distribution.
Distilled water was never an ideal option because over time it will erode stainless steal. Now that the newer irons have a means of dealing with the calcium deposits, there really is no reason to chance damaging the iron by using distilled water.