Our good blog friend DJMH offered up the following on a post by Odyssey:
Because the whole point of supplemental material is that the publisher doesn't want to spend a dime supporting it
This is nonsense. This is not "the whole point". This is peripheral to the real point.
In point of fact, the real reason GlamourMags demand endless amounts of supplementary data is to squeeze out the competition journals. They do this by denying those other journals the data that would otherwise be offered up as additional publications. Don't believe it? Take a look through some issues of Science and Nature from the late 1960s through maybe the mid 1970s. The research publications were barely Brief Communications. A single figure, maybe two. And no associated "Supplemental Materials", either. And then, if you are clever, you will find the real paper that was subsequently published in a totally different journal. A real journal. With all of the meat of the study that was promised by the teaser in the Glam Mag fleshed out.
Glamour wised up and figured out that with the "Supplementary Materials" scam they can lock up the data that used to be put in another journal. This has the effect of both damping citations of that specific material and collecting what citations there are to themselves. All without having to treble or quadruple the size of their print journal.
Nice little scam to increase their Journal Impact Factor distance from the competition.