I think the likelihood that it's a marketing effort is a reasonable, and probable, explanation for these pistols, and I bought the gun with that in mind. From a collector/purist point of view, there are no visual distractions from a pistol that would end up in a GI's holster. It's the same reason I bought one of Beretta's M9 pistols a LONG time ago (the one in the green box with the Bianchi holster, GI belt, etc. - there was, if I recall, a minimal difference in the serial numbers, but it was as close as one could get to the genuine issue M9). As far as I know, the only REAL USGI M17s that have been sold on the civilian market were from about 4 or 5 years ago. Those seem to have been low-numbered guns with FDE controls that MAY have been a trial issue batch, many of which were sold with documentation indicating the unit to which the pistol was issued. Ballpark on those seems to be around $2500 now. I wonder if the FDE controls were the problem: if there was peeling of a coating on those items, that may have led to the change in spec to the black finish on the current-issue pistols.
Comparing the GI production-line M17 and the civilian M18 may indicate different production standards; the slide finish is different, the rear sight/optic plate fixing is altogether different, the slide release is different, etc. I'll say though that the civilian finish on the M18 slide and barrel seems to be slightly BETTER than on the M17, with a lot more bright spots on the M17's barrel.
That being said, it's been the gateway to a revelation for personal defense/concealed carry. I ran a few hundred rounds through the M17, M18, the P320XC, the Beretta M9, a duffle-bag 1986 CZ75, my Glock 19, and a Ruger .380 LCP this past weekend so I could continue to evaluate the best option for carry, and I find still that I can shoot the Sig pistols better than all the rest. A caveat there is that I've put small grip modules on the Sigs, all of which end up easier to control than the Gen4 Glock 19 without a backstrap.