It seems clear that the level of exposure has a lot to do with the severity of what you get from it.
Quite correct, most severe cases in otherwise healthy individuals involve exposure to a high viral load. Such a load may arise during aerosol generating procedures arising from artificial airway support and ventilation. These are common procedures in a HEMS pre-hospital setting. Despite the best precautions flight crew may be exposed, that risk needs to be balanced up by just how critical the service is at the moment. A HEMS crew mostly attending trauma calls in an urban/sub-urban setting are less in demand that a crew whose role is more frequently Specialist Transfer and Retrieval from remote rural locations. It would be sensible to take the former off line, the latter probably needs to continue throughout the Covid19 pandemic.