Can [edit: or should] a scientist's h-index decrease in the case highly cited publications are retracted?
The h-index is defined mathematically based on the number of publications and citations. So the only question is what data source you are using to calculate the h-index. If that source removes publications and citation counts upon retraction, the answer is yes, the h-index can decrease.

However, since this is source-based, you could even get occasional decreases due to other factors. Since many of these systems are automatic, you sometimes see incorrect publication associations and citation counts that can be later corrected, causing the h-index to decrease (I've seen this happening in Google Scholar).

I fail to see why this is interesting, though. Isn't this just like saying the number of publications and citations an author has can only increase, except in special circumstances such as retractions etc.?