I believe the major reason for the voice actors' objections is that for some reason, voice acting in video games is the only work in their field that doesn't pay royalties.
A voice actor receives royalties for television shows, movies, or voiceovers in any other medium, and in any of those media, there're several people who work harder than the voice actor. At the same time, voice acting is acting, and as such is governed by the rules of the SAG.
Basically, the VAs are applying for a new ruling on a field of work that's inexplicably an exemption to the usual rules that they work under. That seems to be well within their rights as workers in a creative endeavor.
What we're seeing here is another one of the growing pains of the industry. I can see both sides of this particular argument, especially since voice acting is becoming such a major part of game development.