That function was served not by SLS but by the formation of United Launch Alliance (a/k/a ULA), which merged the former rocket divisions of both Lockheed/Martin

That function was served not by SLS but by the formation of United Launch Alliance (a/k/a ULA), which merged the former rocket divisions of both Lockheed/Martin and Boeing, operating both their boosters (Atlas V and Delta IV). The formation of this venture was effectively brokered by government, which also let it charge above-market rates for launches, and until recently gave it an additional $1 billion or so per year on top of that to preserve "launch capability and readiness". Like the speciality shipyards that make nuclear submarines, the Pentagon was arranging to make very sure they stayed in business -- though that may be changing, now that lower-priced, American commercial competition has emerged (in the form of SpaceX, with Blue Origin perhaps coming along as well, having announced plans for an orbital booster with capabilities comparable to the SpaceX Falcon Heavy). SLS isn't really suitable for defense satellite launches. It's oversized for the job, and won't be capable of the required flight rates (several launches per year at least) for a very long time, if ever. (Current schedule is years between launches.) It also isn't necessary for launching crew to at least low earth orbit; one of the commercial crew contractors (Boeing) is expecting to send crew up on man-rated versions of the ULA Atlas V.