You are right that all roads are bike routes (bicycles are allowed on all city streets in Oregon except freeways), but the value in Maps' bicycle routing in Portland is that it will prefer official Greenways. These routes have many benefits over adjacent (though still perfectly rideable) alternatives. They have sharrows, no stop signs in the cycling direction, speed bumps, car barriers, and soon, reduced speed limits. They really should, literally, be preferred when planning a route, just to make the ride more enjoyable.
So, in answer to your question, the only reason someone should not use this route is because there is a better one one block over. However, that certainly does not mean the road should be actively avoided, since it is still perfectly fine to cycle on. I just want to take it back to the default state.
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