The reason for this is because the military is fairly conservative about what can be worn with the uniform. Generally mirrored lenses are unauthorized, but you might get away with black iridium. Lenses like fire, ice, and emerald iridium are a big no no. You can get anything from an ass-chewing to a charge sheet for that.
Even if you know someone, you might not always be able to get a pair of SI glasses in the style you want. Not ever PX/BX carries every style.
A pair of SI Minutes just sold on eBay for $72...