They have to use the "100 HP per liter mark" because 1) its far easier to do on a smaller engine....FAR easier, and 2) thats all they have going for them. Frankly, HP per liter is not important, its TOTAL HP, which it always lacks.

M3, M roadster/Coupe, M5, M6 all have 100HP/liter or better, but they also have 330, 400/500 HP too. Theres other cars that do it N/A as well, but why bother? "Okay, you've reached 100/L, congradulations, now what?"

My car running easily surpasses that, and the fact that it HAS HP and TQ are whats cool about it, not so much how much HP it makes for L of displacement.