Looking at it from a promotional point of view, it could be argued that non-Japanese people in a PV would catch the audience's eye more going by what Kikaru said. But as the first post says, it's not uncommon for a PV to have an entirely non-Japanese cast (aside from the artist). It really may just be more of a business thing rather than favoring one group over the other. And there's always the whole western stuff being cool thing. There probably isn't a specific reasoning behind this stuff, they do it because they can.

Dustie - Good points. But Ayumi gets a lot of help from the staff behind the PV and a director wouldn't be needed if she could decide/come up with everything by herself. With the casting process, more likely than not, Avex puts out the casting call (with or without specific notes from Ayumi or the director), the casting staff assembles who they think would be good, and Ayumi can greenlight and/or place the actors as she likes after most of the work is done (because of her schedule). She most definitely participates more when it comes to choosing people she'll work with on a long term basis (like dancers, as was show on Asayan).