Some of the trails above treeline disappear completely, with the exception of following cairns. And, in poor visibility, the cairns can be hard to see as well. Also, several trails blend together, then branch apart again. With poor visibility, not being able to see landmarks OR cairns, it could be pretty disorienting. I hiked mt. washington once on a totally clear day, until I hit the scree slopes on the final ascent. Then fog rolled in. I knew I was going in the right direction because I was climbing up. I was WAY off course though.
These folks, even though the DID get lost, did the right thing. They carried gear to see them through, were at least MARGINALLY prepared for it, and called for help when they realized they were lost.
They will most likely still get charged for the S&R fees, as they will undoubtedly determine that the couple could have walked out on their own in the morning. But, they kept a calm head, and avoided tragedy.