As AK posted; "tier-one" spec ops like ACE, DEVGRU, ISA, AWG(Asymetrical Warfare Group; don't even ask,
) etc have different budgets & procurement systems. There $ comes from a budget source different than the regular or conventional armed forces.
As for the SIG P229/P229R .357sig, it is now in use by a few large federal agencies; US Secret Service, FAMs/Federal Air Marshals Service, even the OIG(office of the Inspector General) for the SSA(Social Security Administration).
The LE/military sales rep for SIG-Sauer could supply a full list of US federal agencies that use the P229 model in .357sig caliber.