They are worried because if they keep it for UC then they can't use the same excuse as last year for not including it in legacy benefits as well ("we can't change the legacy system quickly").
Even a one off payment opens that up to challenge.
And if they do include it for legacy this time then they have no excuse for refusing to backpay legacy for last year as well.
They got away with not including legacy last year because no one took them to court over it, simply because it wasn't seen as the right time at the start of the pandemic/lockdowns.
That will be very different if they give it to UC but try to exclude legacy again this time around, and will open them up again to last year being looked at as well.
They can avoid that by not giving anything extra to UC this year.
To put things right they would have to give legacy whatever they give UC this time, and backpay legacy the £1K missed for last year.
TBH paying (even backpaying) legacy is a drop in the bucket of what they are spending elswhere on Covid measures; but of course that's being spent on the 'hard working' not on benefit 'scroungers'.