As I understand it, crew wishing to accept BA's latest offer had to provide a certificate to the effect they were not union members at the date the offer was declared. As BA have played this whole thing with a straight bat, I'm convinced they would not want to be seen enciting union members to accept the offer outside the collective bargaining unit.
Now, I have little doubt some will have done so and perhaps been a little economic with the accualite. Even so, their certificate will enable the company to keep its hands clean if they did.