The "fucked up" case highlights the fact that ABP injects 14,000+ CSS rules (with only EasyList) in every page
and iframes on a page.
You could as well ask "What kind of fucked up add-on unconditionally injects minimum of 14K+ CSS rules per page? (assuming no iframe)".
The original point is that the approach is detrimental to Firefox the browser regardless of that stress test, and I can understand very well Firefox developers wanting to raise the issue [1], as the browser ends up taking the blame when resources are abused.
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=988266