Moleytt, staff travel is definitely a perk and is not contractual. It can be removed and generally this only happens if someone abuses staff travel itself. My spouse has been with BA a lifetime and I can't recall ever hearing of it being removed as part of, say, a disciplinary, unless the disciplinary was for the staff travel abuse in the first place.
However, as staff travel is available to all staff who don't abuse staff travel, withdrawing it from a group who happen to take part in a legally authorised strike (even if the grounds for voting for said strike are pretty thin) sounds like punishment to me. Yes I know they could have had it back without seniority, but in some ways that's worse, I know how I'd feel if as a family we had to go back to the bottom after years of working ones way up the priority ladder.
To imagine my spouse to have to make a decision to risk that against going against his union colleagues would be agonising however he felt about the IA itself. And if he voted against a strike but the majority vote was in favour, then he firmly believes he should strike. CC have always been a bit half hearted about actually going on strike, as opposed to simply voting to give the union backing in negotiations, in the past many have got round it by going sick, hence the requirement to provide medical of sickness proof this time or it counts as on strike.