A number of Japanese artists does indeed use English titles, but by far from the same extent as Ayu has been doing it.
The only one song that did not have an English title was Sotsogyou Shashin, and it's a cover.
I think it might have a lot to do with her love for the US, and that she simply decided about it some time long ago.
There must be a reason to why she has been keeping it up, and I do think it is because of her having simply decided to do so in the past.
No one really knows
.