The reason is historically all Police HEMS were ex millitary
I'm pretty sure that that has never been the case. Granted the majority of Police pilots are ex military(approx 75%), but, it's not a given that an ex mil guy will get the job, as I'm sure Silsoe Sid could tell you. The advantage that the ex mil have is that they are a known quantity in terms of training and experience.
· Demonstrable flying experience in operations above 5,000 feet above ground level (agl).
The ability for low speed/hover flight at high altitudes will be an advantage for that particular job.