A bit of both, I think. The main difference is that junk mail has a predictable cadence, and capacity can be increased to account for it. On the other hand, chain letters have a very sharp rise time which swamps the available capacity, and a sharp fall time so there's no point in expanding to support it.
The analogy I'd use is cryptocurrency's impact on the GPU market. The sharp rise of people wanting to pay electricity in order to buy into the pyramid scheme has caused GPU prices to skyrocket. It isn't clear yet when either the bubble will burst, or regulation will catch up with cryptocurrency, so there hasn't been much increased GPU production to handle the peak demand.
