Your kidding, right? Try to use Humphrey Bogarts face to sell something and watch what happens.
They are not claiming to be Elvis; they are not using any images or recordings of the real Elvis; and they are selling their own "talent" (such as it is) for impersonation, not Elvis's talent. The only question might be how the show is advertised and if they use Elvis' name and likeness without permission in the promotional materials. As long as they have an ASCAP/BMI license to perform the songs publicly, I don't see how this case can be won. Of course, this isn't in America, so the laws might be different. But I don't think Rich Little has to pay royalties to everyone he imitates during a Las Vegas show.