When this was brought up to FM they said they didn't want to introduce moving parts and a potential failure mechanism.
So they opt'd to keep the battery internal, sealed and with factory installed connectors that they can warranty. I know the board's aren't all sealed up 100% from the factory but I believe that is really FM's intention. Some of the boards out there (cough cough Stroh) get ridden all winter in snow and slush and just keep ticking... other's die on a single 1/4" puddle.
And I know FM doesn't warranty all claims but I assume that number would just go up with a removable battery where the customer can be at fault for changing when damp or somehow damaging the connector.