The idea of the "who cares" or "i don't care" rule is that people used to constantly say "i don't care" to try and be an invincible internet tough guy, when they likely did care and was trying to be smug or cool or something in order to look better than everyone.
Exactly, the "Don't care; don't post" rule only directly applies to the Drama Board. However, the "don't be stupid rule" applies everywhere. Occasionally, saying who cares in a different board can get you banned.
One of the things that really annoys me is when someone posts the "Don't care; don't post" rule somewhere completely unrelated to Drama,
especially when it is something that you wouldn't even get banned for
in Drama.