Since you asked for it, here you go. This is what my "company" says:
B. Service Class
1. General. Government policy is that:
a. Members and/or dependents must be provided coach-class (economy) airline accommodations for all official business travel (including PCS, TDY, COT leave, emergency leave, R&R, FEML, flights over 14 hours, personnel evacuation) unless proper documentation/justification is provided (ordinarily before travel, see par. U2000-A2a) and substantiated to justify premium-class transportation.
To justify premius cabin travel we have to sell our mom into slavery and up our first-borns.