Most 2h swords are quite powerful as well and you can sheath them all but the Flamberge.
Why can you sheath a War Spear which is only a bit shorter (15 difference) but why shouldnt it be possible to sheath an Awlpike.
It makes sense to sheath a war spear though as it is a weapon used by hoplites. You cannot use an Awlpike together with a shield (anymore!) though. That's actually a reason to make it un-sheathable.
However the Awlpike is a nice weapon to be a secondary weaponry it seems. You can be a lancer on horse and take out the awlpike once on the ground.
There's a second Awlpike, the long Awlpike which you cannot sheath so that's actually the "pair" weapon of the Halberd I guess?
Except for Lances and Long Spear/Pike there are only a few weapons you cannot sheath in the Pole Tree. Just noticed you cannot sheath the Aswhood Pike either (which is actually a good weapon for hoplites), so screw my argument above.