This should not be a surprise to BALPA, if they had seen the deal structure proposed.
It seems that BA have told DLH that they consider the pension scheme to be akin to debt for which they will not assume any liability or future liabilities.
DLH may have been in a position by which they could have legally walked away from the complete (£180m?) liability, albeit the
PR implications would have been massive. Instead DLH have negotiated with BA and the PPF an £85m one-off payment, to try and keep the workforce at least half motivated and avoid any potential legal challenges to DLH for full recompense.
The PPF was aimed towards helping those pension schemes (i.e. unsecured creditors) affected with companies (and parent companies) that have gone bust e.g. Woolworths etc.
Whilst BMI was in no way a going concern, it was acquired by DLH which knew full well of the huge pension scheme liabilities upon signing its various purchase agreements and from the sounds of this deal, Lufthansa (a fully solvent group) is able to walk away from this liability.