Spell casting is described on page 105 in the standard rules. Under "Automatic Spell Casting", it is stated that "If a spell is cast automatically, no static maneuver is required". Later, it is described what causes a spell to be cast automatically. I think in this case, the following case is what's interresting:
3) The spell is not cast as a snap action or if the spell is an instantaneous spell cast as a snap action
So, if you cast an instantaneous spell as a snap action, that spell is cast automatically (assuming no other penalties, like using armour, for instance). And for automatically cast spells, no SCSM is necessary. So there is no maneouver, and thus no maneouver to apply the -10 for snap action to.
A related question: Do you apply the "ordinary" penalties for static maneouvers to SCSMs? We do, but only if an SCSM is called for (so, having a penalty to static maneouvers does not force an SCSM, but if you DO have to make an SCSM, we apply the mods). The most interresting here is probably -20 for being in a "melee environment" and the -30 for being "in melee". So in such cases, and spells better be cast automatically.