Safety Concerns
The problem I see with your approach to the issue is that you're talking about the principle of removing staff travel generally, and whether this constitutes "punishment". BA have already effectively conceded the point, though. They offered reinstatement of staff travel, but without seniority, as part of the deal to settle the overall dispute. Unite/BASSA rejected this following a consultative ballot. Note that the Union could have ignored the ballot result and accepted anyway. So the reason striking staff no longer have staff travel privileges is simply down to the fact that members declined the offer including its reinstatement in a ballot, and the Union chose to follow the result even though not strictly obliged to do so.
There is plenty of evidence in the public domain stating "we'd have accepted [the deal overall] if full staff travel rights had been offered". So staff cannot now claim their rejection of the deal to reinstate staff travel was merely incidental, and that the main reasons for rejection were unconnected with staff travel. On top of that, one of Unite's main stated reasons for not recommending acceptance of the offer was that it did not include reinstatement of full staff travel rights. Therefore the reason why strikers don't have any staff travel rights now is because they declined them. BA offered them back. I just don't see how Unite can construct a valid case against BA on the overall principle of removing staff travel, because BA actually hasn't done so.