It’s a rare round.
.357Sig isn’t the least bit rare. Many Fed agencies and municipal PDs use it. Think .40 S&W with rebated rim and .357 business end.
The Feds are buying many calibers in their armament program. .40 S&W, .357 Sig, , 9mm, and most of them hollow point slugs. H P rounds are illegal for military use. Seems like the decision makers are reading the literature on what is the most popular round of the moment and then buying it. Not much thought on what gun is going to shoot it.
USDA just put out a request for an MP5 type sub-gun that will launch the .40. I guess they felt a little out gunned in Nevada with their Glock 22’s against some serious firepower.