Of course they are disappointed. Great Rojava was seem in their hands already.
Kurdish lands will shrink naturally back to their real size.
The US wanted the SDF to clear Manbej then withdraw, that's how they got the agreement of Turkey on the op. Unfortunately the Kurds overplayed themselves this time by trying to keep those areas against US wishes, so Turkey got involved and the US had no choice but to back them. US will still support the SDF against ISIS east of the Euphrates, but between SDF and Turkey it's a no-brainer as long as a choice exists. US interests aren't synonymous with Kurd interests and vice versa, even if there is overlap.
Even with this check on SDF expansion though, the Kurds in Syria were saved from total destruction from ISIS by the US at Kobane, and have expanded their control greatly with US support, that hasn't changed.
:edit: looks like I spoke too soon, if SDF is trying to stay involved west of the Euphrates that's a really stupid move... The US wanted Turkey's involvement more than the SDF, because Turkey didn't get involved the US supported the Kurds...
Clearly the US supports this op, as the US wanted Turkey to get involved from the beginning, just as clearly Assad is against it.
So far i'm surprised at how adroitly the US gov is playing the region for its interests, I suppose a decade or so of intimate involvement can give one a good experience of the region.