Americanisms... ruining English everywhere

The Superficial is one of my fave gossip sites, and I guess I shouldn't be expecting perfect sentence formation from any online gossip rag, but generally they don't do too bad a job.

This recent article, however, contains one of my pet hates:

The attendant had no idea and could have really cared less.

The phrase is "could NOT have cared less". Not "could have cared less", which actually indicates the attendant was indeed a fair bit more strung out than they're attempting to portray.

The only people I know who regularly get this wrong are Americans. Is it mere laziness? Or is there actually a true lessening of intelligence once we cross the seas into Yankie land?

Sheesh. Learn it, people, or leave it alone.