I think that higher rate relief is not unfair, but maybe for a different reason- simply because they have paid tax at 40% and so any relief should be at the same rate. I no longer pay tax at 40% but the rate does kick in pretty early nowadays and many people who do not consider themselves well off (teachers, skilled labourers doing overtime, etc) now find themselves paying some tax at 40%. And some of those better off employees who get tax relief at 40% may also have to pay tax at 40% on some of their pension.
Having said that, I think that the tax benefits of pension tax relief are only worthwhile for higher rate taxpayers. For a basic rate tax payer they may get relief at 20% but as the eventual pension income is taxed at 20% there is no real benefit, so (when coupled with the existence of means tested pension credit) it's not surprising so few people make enough provision for retirement.