So far as I know the SCAR Mk16/Mk17 family of weapons was developed to replace a slew of limited use/purchase weapons from a variety of manufacturers to include Colt and Knights Armament, such as the M4 series, Mk11 series, and Mk12 series among other variants. The Mk17 has already been in use by USSOCOM subordinate commands for some time now. Whether or not it is "standard" is open for debate. Come tomorrow, who knows what they'll be carrying.
Manufacturers like stating that their weapons have been "adopted" by this, that, or the other special forces group whenever a handful of weapons are purchased and field tested or actually fielded in limited quantities by some subordinate unit of USSOCOM. If anything could be considered "standard" across all services, to include USSOCOM's subordinate commands, it would be the Colt M4 series and Colt M16 series.
I don't claim know anything about what USSOCOM does or does not do, but I'd wager that most commands have probably already given up the Mk16's and HK416's or put them into storage. The chief complaints against the Mk16 and HK416 were the same as for the M14EBR. It's too heavy. I find it instructive that even USSOCOM personnel, who are arguably in much better physical condition than most other Americans, still have issues with the weight of the weapons they are using. However well funded it may be, even USSOCOM has a budget and all of the special purpose weapons cost more than the M4.
So far as I know, the Mk17 rifles are made in America, unlike the civilian SCAR 17S rifles which are imported from Belgium, and cost much less to manufacture than the new manufacture M14's from SEI or the imported SCAR 17S's. I have no idea if James River Armory or LRB produce M14 receivers or bolts for the government, but SEI has in the past and probably still does in some limited manner.
I've personally asked several Navy SEAL's about why they carry M4 variants, given that a couple of them stated that they can pretty much carry whatever they want, with even shorter barrels than the standard M4 barrel and the effectiveness of the short barrel weapons. The answers were pretty simple:
1. Bigger weapons weigh too much given everything else I have to carry.
2. Longer weapons have a tendency to snag on equipment and the interiors of aircraft and vehicles while entering and exiting.
3. The balance of the weapon and light weight make it extremely fast to point and shoot.
4. While it may be true that the shorter barrels are less effective at range, most engagements are ambushes or surprise/unavoidable contact at close range. In those cases, it doesn't matter what I shoot them with. If they don't go down, I'll shoot them again.