Sorry, but Oregon is different. They treat the endorsement just like helmet laws - just because you don't need one in Idaho, doesn't mean you can ride in Oregon without one - the endorsement is the same.
In a similar vein, you bring your ATV to Oregon without the new Oregon "safety license" and ride, or let your (under 18) kid ride one without you having one and being within 100 feet of them, and the law states they have the right to impound the vehicle (ie: keep it).
This is Oregon's way of saying they only want you to be able to walk or ride a bike. Nothing else is "green" enough...
An endorsement is differnt that a Helmet law. An endorsment states, nothing more than, you are licensed to operate that vehicle configuration in your home state.
A helmet law, is like a seat belt law, Cell phone law, or no splitting lane law. In the those cases, is does not matter what your are licensed to operate. Those laws require that you meet the individuals states safety laws while operatiing that vehicle, int the relative state you are traveling in.
We can take this even further. Vehicle registration varies from state to state as well. Trailering also varies. We can go on about the differences for days, and many of us will still be confussed.