There is no rule that says that the last CT can't camp. The current MOTD and its previous iterations have all had the line, "CTs can camp if it helps their team, but the last CT cannot not delay the round." Would you expect me to just run out of the scout tower and get instakilled?
Except that would make the scout tower situation and cappn's example violate the rules, even when you said yourself that I shouldn't have been slain. There is no reason to add "Last CT can't camp" to the rules. If a last CT is camping but not delaying, like in the scout tower situation, they shouldn't be slayed. If the CT is just sitting in some random vent, then he/she are delaying and can be slain.
I agree. Just saying "Last CT can't camp" and slaying is both lazy and illegitimate.