That's a cool find.
Kuu has a bunch of cover songs and some she shares with other artists, but it's pretty common for western songwriters to sell their songs to asian artists and companies, sometimes even in both worlds (in the western and Asia, that's what happens to Can We Go Back, which isn't a cover, it was sold to Kelly Clarkson and Kuu).
And what's the difference between having japanese songwriters or western? The only difference is that songs are maybe written for that specific artist, but you can't always say if thats the case.
And by the way, Kuu composed a few songs, Milk Tea and the Introduction for Kingdom for example are composed and written by her. And well...an artist really should just do what he or she is able to.
I love my Kumi Kuu, even if all of her songs would have been cover songs. She always gives her touch to the songs (own lyrics, rearranging and such) anyway.