In some cases, on SMP when inside a protected area (an area in which the destruction and placement of blocks is prohibited) plant-based blocks (tested in a jungle with jungle leaves and vines) can be destroyed. However, other blocks could not be destroyed.
This server was most probably using a plugin to protect this area (WorldGuard) and was a Bukkit server.
This ticket is invalid as it relates to a modded or 3rd party client/server.
Any non-standard client/server build needs to be taken up with the appropriate team, not Mojang.
Any plugin issues need to be addressed to the plugin creator.
This site is for addressing issues related to the base unmodded MineCraft.