They couldn't remove the UC icrease now without ammending the current law.
That's because of the way they did it by adding it to the UC Standard Allowance in the anual uprating.
It's not widely realised that the anual benefits upratings require an ammendment to the law(s) each year.
That's also why they said it's for 12 months - until the next anual uprating/ammendment(s) are due.
If they wanted to remove it now for pre-covid UC claimants only, but leave it for new claimants, then that would need a much bigger change to the law.
Which is what they were trying to avoid in the first place by doing it for all UC claimants.
And, just for reassurance of those on UC, they couldn't make any ammendment retrospective and claim back what they have already paid.
They were fortunate (???) that the anual uprating was due just as they wanted to do this and so it was a simple matter to change the amount that UC Standard Allowance was being uprated to.
If it had been any other time of year they would have had to make another ammendment to UC law in order to change the rates.
(So maybe not so fortunate, if they had had to think about it more they would probably have done it differently and avoided the problems with discrimination).
They are now saying that for some reason it would not have been as simple to do the same with legacy benefits.
I don't see why not, the Personal Allowance rate in legacy benefits was changing in April anyway so all they would have had to do was alter what it increased to, exactly what they did with the UC Standard Allowance.
Even if it was more dificult to increase the PA in legacy benefits the point is that they can't (legally) discriminate against a set of claimants just because it's easier.