Perhaps they are thinking about the costs? 8 Amazon Basics rechargeable D-cells are going to run your $90 (plus the charger itself)... They are 10,000 mah.
For $37 you can get 16x 2000 mah Amazon Basics AA cells and 10 AA-D adapters
Sure, if the former are going to last 4 hours and the latter only like ~50 minutes, then I guess that would suck if you are commonly using the speaker for several hours at a time.