Even perfectly legal activity can result in undesirable retribution. For example, some rich company could decide to sue a person in response to a negative review. Whistleblowers will almost certainly face retaliation if their anonymity is not guaranteed. Anonymous discussion boards are inherently more free: because there is no identity associated with opinions, there are no consequences for being wrong or politically incorrect and this leads to people speaking their minds without fear.

Also, not everything that is illegal should be illegal. Civil disobedience is impossible in a monitored society.