Yes, player's rolls can explode downwards (i.e. implode), but that implosion has a average maximum limit of -19 or so on the dice, where as for exploding, there is no limit to the potential on how high it can go.
Remember, when dice explode, that means that it rolled 10. When the dice implode, that means it rolled a 1. A 9 point difference between the two...
Some other effects of using such an option are as follows:
1) It becomes increasing unlikely that the NPC will ever get/use any sort of Boon Point for increased or special effects when they make an attack. This option would effectively limit Boon Points to ONLY the PCs. It also eliminates the possibility of the NPC ever fumbling and earning a Snag point...
2) It would require keeping an extra set of numbers for the NPC (the normal DEF and AB for use against the players' attacks AND the modified set for NPCs' attacks).
3) It requires using a different resolution method for NPCs, which kinda breaks the nice "single resolution mechanic" theme Novus was trying exploit.