The opponent did set a defense: just not a complete defense.
Would it really have been better if your opponent placed 1 level 1 toon in each defense slot other than ships?
Your opponent would have won handily, and put no more effort into placing defenses and you'd still not have gotten progress on most of your quest items - it's hard to resurrect a toon if their defense can't kill you!
If people don't set defenses they lose. If people set some defense, but not a lot, saving their best toons to kick your defenses all over the map, well, that's apparently good strategy.
At some point you have to let people decide what they're going to devote to offense and what to defense. The current arrangement sets a decent balance.
