Kamel, as far as I'm aware those going in their own time are those who don't want to wait to be told to go, and want to be able to be rostered for US flights right away.
Those waiting on main fleet are getting rostered days off to attend (as per one case I know of in the last 2 weeks)
If they're not short of crew with visas on one fleet or the other, then of course they're not going to spend the time to organise it all...