I don't see where they in any way ban Polymer Tipped Bullets for muzzleloaders.
(Unless you're referring to using smaller caliber Polymer Tipped Bullets in conjunction with Sabots? Then those aren't allowed in Oregon)
But then, on the other hand, there's really
NO need to use them!
Unless you're looking at something different from what I'm aware is out there, Polymer Tipped Bullets are generally for being used in Centerfire Rifles to prevent damage to the tips occuring in the rifle's magazine from recoil of the rifle.
More recently Hornady's introduced SOFT Polymer Tips that now allow Tubular Magazine (mainly Lever Action) rifles to use pointed bullets.
Previously, they always had to use flat pointed bullets to prevent discharge of cartridges in the magazine by recoil inertia setting off primers from the tips of pointed bullets.
The object of having these Polymer points has always been to keep the pointed tips POINTED.
And these pointy bullets work best in high velocity rifle cartridges and firearms.
But Muzzleloaders ARE NOT high velocity firearms and they
DON'T NEED pointed tip projectiles. In fact
they fly better at sub-sonic velocities with ROUND NOSED projectiles, just like those you see on .22 Rimfire ammunition.
Also bullets for Muzzleloaders benefit from having fairly wide noses on them. It aids in spreading out the force required to push the bullet down the bore with a ram rod.