Its not a stupid question.  A miner who can produce a double spend block can also potentially just produce a "legit" block.  25 BTC * $120 = ~$3,000 per block.  There is a real COST for a miner to attempt (with <51% of hashpower) to extend an alternate chain.  The risk is that the rest of the network builds a longer chain and not only does the miner not successfully double spend any blocks produced in the attempt become worthless as orphans.

That cost is what gives the network security.  That cost also means that waiting 6 confirms before transfering a $20 game about as silly as keeping a 1 oz gold bullion in a $50,000 safe guarded 24/7 by fully armed security detail.