The free 10 WoF spin was a recent promotion they sent to all their users' email addresses not just premium users with FUN token balance; and it says that if you complete 10 free spins you will automatically receive the 10 WoF spins.
I haven't receive that email. So, I think it was sent to all users.
The mail was probably sent to inactive users to encourage them to become active again.
And why is the "Big Wins" thread on Bitcointalk packed with newly registered users making posts?
That's normal.
There are many freebitco users that don't have account here in bitcointalk and once they hit a number from 9998 to 10000, they create an account in bitcointalk to post about their big win and claim the extra reward.