"Now spreading round the globe"? Old news, FR/U2 et al have been doing this since their inception (well in the case of FR, since their rebirth).
I could be wrong but it came to the US later when some ULCCs adopted it, followed by larger carriers. WN (Southwest) still doesn't do it.
It's only a cartel if they sit down and agree it. Parallel pricing, where they watch each other closely and then match, is normal market behaviour and quite legal.