This restriction is a proactive anti-harassment measure. Without this cooldown, a malicious user could block a target (to hide their own activity), unblock to post harassing messages, and re-block immediately to prevent the victim from responding. By enforcing a 48-hour window where the user cannot re-hide themselves, Facebook ensures the aggressor is visible and accountable for their actions during that timeframe.
This restriction is a proactive anti-harassment measure. Without this cooldown, a malicious user could block a target (to hide their own activity), unblock to post harassing messages, and re-block immediately to prevent the victim from responding. By enforcing a 48-hour window where the user cannot re-hide themselves, Facebook ensures the aggressor is visible and accountable for their actions during that timeframe.