The average fuel burn per km for a particular aircraft type starts (at extremely short distances) extremely high, then falls sharply as distance increases, flattening out at what is that aircraft's optimum sector length. Thereafter it increases slowly - the aircraft is burning fuel in order to carry the fuel it needs for the longer sectors.
So for a flight from, say, London to Singapore, the fuel burn per km is greater than for a flight from London to Dubai, or from Dubai to Singapore. So assuming that Dubai is more or less on the great circle between London and Singapore, you could end up burning less fuel on two legs than on one.
Apart from that ... 'render it illegal' - how? who? under what jurisdiction?