Unfair because it is subsidising those who choose EVs over those who can't (or won't), by paying for their fuel.
It is arguable that the tax free bike purchase schemes are unfair on those who for whatever reason can't use a bike to commute. But those schemes are government sponsored, they don't cost the employer much at all.
It's subsidising a choice that, agree with it or not, is deemed to be beneficial to society and the environment.
It's employers' choice to provide perks to their employees depending on cost and predicted benefit (recruitment, employee satisfaction, staff retention, green credentials etc). Other perks include free parking, for all cars not just EVs. As we're near the city centre that's quite financially attractive. Again, arguably unfair on those who have to take public transport to work.
My employer also have a salary sacrifice car lease scheme but only for EVs. I've not taken advantage of it because my perception is I don't do enough miles for it to be beneficial and at the moment am happy with my current car. I need to do the sums on this though.