It's not a case of black-mailing anyone. The right to strike is still there. Remember that staff travel is refered to as either a perk or a privilege - defined as an employee benefit. Why should anyone receive benefits for causing the company harm? (whatever the situation).
It is also defined as an incentive, this is given if you do something. That something could be "do not damage your company by striking". You have therefore not achieved the conditions required for the incentive, therefore you don't receive it.
It is not contractual so the removal of it is in no-way black-mail.