Or to put succinctly in terms of OS4 development yearly milestones:

1999: MorphOS looks cool, hope it is AmigaOS4
2000: AMITHLON AND MOPHROS IS PIRATE
2001: AMITHLON AND MOPHROS IS PIRATE
2002: MOPHROS IS PIRATE
2003: MOPHROS IS PIRATE
2004: MOPHROS IS PIRATE
2005: MOPHROS AND AROS IS PIRATE
2006: ...but stealing IP from Amiga Inc is ok since they're bad.
2007: MOPHROS AND AROS IS PIRATE
2008: MOPHROS AND AROS IS PIRATE
2009: MOPHROS AND AROS IS PIRATE
2010: MOPHROS AND AROS IS PIRATE
2011: ...wait, we have no developers left. Time to co-operate and bury the hatchet?
2012: AROS guys, nobody woudl have used your code anyway so it's ok for us to take it.
2013: What do you mean MUI4 belongs to you? It belongs to AMIGA!
2014: "EVERYONE IS PIRATE EXCEPT US" - Ben Hermans
2015: MorphOS guys are being selfish for not letting us use their code, even though they stole it in the first place. So it's ok for us to steal it back!