The case for the levy is that it helps to pay for the pension; it is not consistent to apply it to the pension itself (any more than pensioners make contributions to the scheme once they have retired). How could a levy be applied to new pensioners, and not to existing pensioners?
I think a political choice was made not to hit pensioners in the pocket. For now. Over the next couple of years, unless things take a surprising turn for the better, further steps will have to be taken to bring our budget closer to balancing. I don't think that I, as a public service pensioner, will escape scot-free (isn't it a nice novelty to use that expression in its literal sense?)