I agree, if there was a standard for the terrain rating that says that a 1 Terrain must be wheelchair accessible, then it should definately be followed. The way I see it now, it is just a quasy "guideline", and nothing seems to be set in stone (atleast from what I was reading on the web site) as to a 1 rating MUST be wheelchair accessible (or handicap accessible). And being wheelchair accessible, the cache would have to be at a position where I could reach it from sitting in my wheelchair, and not on the ground.
Then again if I was confined to a wheelchair, odds are I would not pick geocaching as a hobby (ok I am sure some do). I would instead put my faith in God in the hopes that one day I could walk again, despite what the doctors say.
As a solution, which I believe has been suggested before, is to have a check box that says that the cache is wheelchair accessible, or if a user does pick a 1 for the terrain, have a message pop up or something indicating that a 1 is intended to be for wheelchair access, and if not the system woudl change it to a 1.5. But as it is now, it's up to the individual cache placer to define what they feel a 1 is in their eyes.