The problem with patients with psychiatric disorders is that it's all dependent on the patient, the disease, and recent history. Just the presence alone of a psychiatric disorder shouldn't ever be enough to demand, dictate, or provide any form of restraints (To show the, erm, insanity of that extreme, would you sedate a patient diagnosed with alcoholism or someone with a phobia and nothing else? Both are in the DSM).
I think another issue with EMS and patients with psychiatric disorders is that the EMS training seems to be confined to no restraints, 4 point restraints, and chemical restraints. While this is fine for the acutely ill 911 patient exhibiting active threats to themself or others, it's a completely inappropriate mindset for interfacility transports. For the "might become violent" (especially the ones who are compliant and cooperative at time of transport), did you consider alternative restraint methods? Maybe a vest style restraint (i.e. poesy vest) might have been appropriate. Another thing to think to consider is what are the trigger points (if any are known) and the patient's recent history. A patient who was combative a week ago prior to starting (or restarting if non-complaint) medications might not be a likely threat today because of the medications. Similarly, a patient who is being restrained 'just because' might view the application of restraints as a threat against themselves, and respond in an appropriate manner.