Why does one bulk Trello action count as 250 automation runs?

That page says "Automation run: Every time an automation button is pressed, or an automation is triggered, it counts as one automation run."
It's not clear and open to interpretation.
Anyway, I need to wait until Tuesday.

Arek's answer is right, and the reason that sentence reads ambiguously is that the counter is on triggers rather than on clicks. Your rule's trigger is a per-card condition, so one press of Move all cards in this list generated 250 separate trigger events.

Your own log settles it. Runs 250 of 250, operations 250 of 2500. That is one operation per run, which is a single label action fired 250 times. Had one run swept the whole list you would be looking at 1 run and 250 operations.

The docs do separate the two ideas, though not in the sentence you quoted. A board button is described as one that reorders or moves all cards in a list matching specific criteria, and there is a distinct limit named the number of cards a single automation run will affect, which only means something if one run can touch many cards.

So the shape that costs you one run is a board button doing the bulk work, not a rule each card trips on its way past.