I think for many it's liability. When you're working from an active medXpress application you're exercising the privileges of an AME and serving as an agent of the FAA Administrator. You're simply evaluating and determining if they meet part 67 or not. Certainly some nuances there, but much of the liability is on the FAA rather than the individual examiner. The examiner can always defer the case.
On the other hand, there is no provision within the roles and responsibilities of a AME to consult an airman for the purposes of future potential medical certification. As such, an AME providing guidance to an airman is doing so outside of their delegated authority. That doesn't mean the FAA prohibits AMEs from conducting consultation appointments, but that they are doing so not wearing their AME hat. I suspect the concern for some AMEs is that if they meet with someone and advise him or her that they are likely unable to obtain a medical certificate, there's a perceived liability if that person pursues a sport pilot certificate and later crashes. Arguably the plaintiff's attorney will assert that the doctor agreed to a consultation in order to bypass the medical certification process and and was complicit in encouraging the person to go sport pilot even though he had a medical condition that the FAA would not allow. It's outside my scope to say whether that's a valid concern, but plenty of AME's don't see a good incentive to enter that realm.
