The reason for this being, as I stated before, bukkit not sending BlockPlaceEvent. Unless there is a plugin that circumvents bukkits block events, and finds its own, there should be no way any plugins properly stop block placement.
That means any plugin using bukkit will not receive BlockPlaceEvent, (such as here (worldguard) : https://github.com/sk89q/worldguard/blob/master/src/main/java/com/sk89q/worldguard/bukkit/WorldGuardBlockListener.java#L476) so the plugins cannot stop or even know that blocks are being placed.
I'm not aware of any open source code for tekkit, thus I can only speculate , but this probably stems from tekkit fiddling with some part of bukkit, and forgetting/stopping BlockPlaceEvent.
I would start by finding how bukkit stores the events internally, here (https://github.com/Bukkit/Bukkit/blob/master/src/main/java/org/bukkit/plugin/SimplePluginManager.java#L466) , debugging by printing off all events directly from bukkits registerEventListeners, seeing if bukkit is grabbing them but not sending them, and if it isn't grabbing them checking to make sure the mc class files within tekkit have not been modified to stop bukkit from recognizing BlockPlaceEvents. I notice packet61 "World Events" has been changed somehow between tekkit and bukkit, for example. Perhaps thats just a version difference though.
Tekkit certainly does have a customEventListener class, so perhaps that is the reason.
Who really knows though?