You'll find that most every arkfall shows you that arkfall's objective at the top of your screen. For some types, the objective is to simply kill repeating waves of enemies. For others, it's to destroy a target -- either multiple smaller stage ones, or a single large one.
The objective for the arkfall you seem to be describing is: Destroy the Crystal. Should say so, right at the top. If it's a hellbug one, you do that by shooting weak spots. If it's mutants/raiders/99ers with that objective, they'll either drop superweapons for that purpose, put down explosive fuel barrels, etc. Your EGO will even remind you, often incessantly, of the current method available to destroy the crystal faster.
Either way, that's THE goal for that arkfall, and that's what those folks were doing. It's unfortunate they flamed you, but it's possible they might have been a little taken aback at someone complaining about them completing the arkfall goal.
BTW, re: keys. The little cylinders you're describing are probably the raw ark
salvage things drop on the ground, whether you kill them in arkfalls or anywhere else. They weren't keycodes, as nothing physically 'drops' those. You get them as auto-rewards, and you'll get pretty much the same set amount by taking part in all arkfalls now -- 2-3 keys for minors, 5-6 for majors.