I have a bit of a better view of this missing feature now. First of all, a straight-up DELETE of users is a risky operation b/c that results in actual removed tuples and potential relational DB problems. So maybe allowing for that in the GUI admin console in bulk is not a great idea, and there is some wisdom requiring this operation to be performed by an admin that can understand/handle the SQL operation.
However, what should exist, is a bulk DEACTIVATE option, which is what I really needed when I first came across this request. And in that case, I get all conspiracy theory about it, because Jira licenses are priced based active user count, so a bulk deactivate is a feature that reduces revenue. That seems obvious enough that I probably shouldn't call it a conspiracy. How often do any of us build things with revenue-reducing potential?
We get attacks on a regular base. I can recommend to enabled Captchas. Feature would be handy though.