It is an easy concession for the Government to make. It aim was to raise more money and discourage car use. The rising price has already done this, so no need for the rise in duty.
Rather like a robber taking money out of your wallet but leaving you with enough for your bus fare home. Of course, you'd show undying gratitude, wouldn't you?
I suppose in this current economic climate, the government has to be seen to be doing something, but it's not losing out. It's making fortunes from the tax it receives from rising petrol prices - but as soon as that changes, it will whack its 2p - or more - on.