Think about this the next time you leave the US on vacation. You have no way of knowing if you have been somehow designated an "enemy combatant".
IANAL, but I'm pretty sure from the wording that the "opaque case" idea is a specific reference to existing judicial opinions. It's an important principle, even for cryptography: it says that the government is not allowed to break your encryption merely because they can. By encrypting your data, you did more than just secure it, you also made it private, in the same way that an opaque container makes the contents private, even if the lock fails or can be broken or picked.
I don't think wrongful blocks are very common. In a typical case, I would think a "contributing normal user" would drop the admin a polite note requesting he/she reconsider, and would be quickly unblocked. The conditions under which admins are allowed to block users are pretty well established—they are not supposed to be blocking people arbitrarily, and are supposed to try to be impartial. There are appeal mechanisms available if the blocking admin is unwilling to reconsider. Wrongful blocks are more of a problem with new users, who may not know what to do.
Note that admins do not permanently "delete" or ban a user for a single offense. Even outright vandalism typically gets a 24 hour block the first time, and even then only after several warnings. Admins who abuse their tools can and do get de-admined.
With your bare hands?!?