Been drinking decaf (instant + filter) for a while, due to causing anxiety and crashing badly after. And it’s only when someone doesn’t have decaf and you drink normal coffee (exact same strength, even if made by self) that one realises how bitter the caffeine itself tastes. (Don’t drink sugar, just milk.)

So perhaps they want that bitter bite, which decaf doesn’t do naturally, which the caffeine being the most bitter part of the drink.