Legal it is.
Fairness does not come into it as far as 'fair' means paying attention to personal preferences is concerned.
When a surplus exists within a trade group/skill group, it is up to the employer to decide who stays and who goes, as long as they do not breach any sex or racial discrimination rules. It is not uncommon to have people wishing to leave being refused, and those wishing to stay being asked to leave.
I have no idea of the particular circumstances to which you refer, nor do I need to know, but typically this happens when the group wishing to go have higher long term benefits to the employer and may be expensive to release. They can in industry frequently be the least flexible and a royal PITA which may seem a contradiction but usually is not.
Those wishing to leave are often less experienced, cheaper to release and of relatively replaceable quality. They may also have jobs lined up due to their less specialised background, lower cost of employment and flexibility, but importantly in industry, they are hoping to be paid to leave, and to walk into a better job.
While none of this may apply to the OP, it is common elsewhere and shapes the way organisations handle staff level reductions.
Nobody could say it is completely fait but the employer has to be able to continue in business, and their choices are made to support that rationale - not the preferences of leaving staff.