It was said before that AES could be purchased by public safety agencies when applied for in writing stating their case why it was needed. In this case, St Thomas bought a new system a few years ago with the full intent of it being 'digital and secure'. They messed around with EP more than once only to have unsatisfactory results (EP has many well known issues including missed transmissions etc, which are obviously crucial the the public safety world). From what I've read elsewhere AES doesn't seem to have these downfalls, so I'm guessing that's what they or their radio provider did. Got ahold of Motorola and said look.... we were promised a secure system and EP doesn't work. What can you do for us? Case stated.
Not sure why they are so anal about it. Hytera has no issues selling it (Canada Border Services Agency in Niagara Falls recently rolled out a Hytera DMR Tier III system with full AES).