I suspect (although I've made no effort to verify this) that some standards organisations were set up in the days of paper copies and secretaries and typists, when they needed a 10-storey office building [1]. So naturally they set up a pricing structure reflecting those expenses.

Then they kept the employee numbers and pricing structure due to institutional inertia.

[1] https://goo.gl/maps/MK3C9AUgzV16Svem8

There's more to an organizations overhead than just the web servers. There's staff, reviewers, publishers, editors, etc. Of course, it's probably still most likely over-priced, and I have no insight into ISO's operational funding or internals.

Do ISO handle proofreading, editing and so on? My impression was that they mostly did publishing, which has become less and less of a cost center.

I think that's exactly correct. It's more than inertia; they are being paid to not update their polices, so they're incentivized to not do so... even though their policies inhibit the use of standards (their purported goal).