I feel like everyone should have the opportunity to save for their retirement in an equal way - and I think the 20% or 40% relief is a bit of a red herring, it should be thought of as tax free savings. This gives people (anyone!) a chance to save as much as possible whilst working, which in turn is invested (good for the economy?) and grows over the years, and they then pay tax on a much larger sum when they need it later in life (which again, good for the economy). It also puts less strain on the state if people have their own money when retired.
The "40% relief" comes from the fact that not all pensions are taken pre-tax, for example a lot of auto-enrolment pensions managed by NEST are taken post-tax and the 20% "relief" is claimed back by NEST on your behalf (e.g. you pay in £100, they will claim £20 for you). If you are a higher rate tax payer you can contact HMRC to get your tax code changed to take into account the extra 20% so you pay slightly less tax on your PAYE earnings.
Using an example above, if your pension contributions are taken from money that crosses a tax threshold, you are only eligible for the "40% relief" on the portion that is subject to 40% tax. For example if you put £500 into your pension each month, and only £250 of your salary was subject to £40% tax then you could only claim the additional 20% tax relief on £250 of your contributions.
As gramercy mentioned, if people who pay tax at 40% are made to pay 20% tax on their pension contributions, then they will be paying 20% on the way in, and 20% on the way out, which hardly seems fair compared to a base rate saver who pays nothing going in and 20% coming out.
I admit I am biased a bit as I am in the higher rate tax band, but I feel like we shouldn't be targeting these kinds of tax initiatives at the people who are doing a bit better than average because it isn't those people who are harbouring all the wealth that needs distributing - we should be targeting the mega wealthy. It feels like this topic is another way to turn the everyday people against each other rather than focussing on the real problem.