IMO not allowing shorting will make for a less stable and less liquid market, not a more stable one. On the other hand I can see their concerns in that if this thing gapped to 50k over a weekend there might be some real counterparty risk of which they might be involved in.
It is my understanding that the counterparty risk is with the exchange. What the exchange requires from the client is the maintenance margin requirement posted "overnight". From the FCM, they require 8% of the client's overnight margin requirement. Ultimately, it is the creditworthiness of the exchange that is your counter-party.