1. If necroposting isn't allowed to prevent spamming why aren't very old threads lock?
I can't see any unofficial rule that prevents necroposting.For the unofficial rules, refer to this thread: Unofficial list of (official) Bitcointalk.org rules, guidelines, FAQ
There's a related reply from the author regarding the topic:
Well, slow down at least. 
Regarding necro posting, if the necro post is substantial (doesn't break the rule especially rule 1 and 2 (2014/07/21 version)), then it's OK.
Regarding necro posting, if the necro post is substantial (doesn't break the rule especially rule 1 and 2 (2014/07/21 version)), then it's OK.