The punishment is pretty ludicrous really.
Basically, because F1 teams are under the parent company microscope (this year owing to the financial crisis), and F1 doesn't want one of the marquee teams bailing them, Renault haven't been fined.
A fine is completely justified imo. It was a serious fraud which affected the sporting result, it endangered spectators and staff, and earned Renault gobal plaudits (for the win) and sponsorship clout in the order of tens of millions of dollars.
It also doesn't cover the fact they earned points fraudulently which meant x millions of dollars from F1 money pool to the direct detriment of other, smaller teams.
I hope the affected teams and the Singapore Grand Prix organisers pursue Renault for financial damages over this.