As I march into the sparta territory, I found a lone squad of catapults, sitting right next to a group of neutral sheep, as if the AI is trying to capture them with catapult.

My guess is the lack of option, as sparta only have 2 types of land based combat unit. When my army approach an enemy city with low garrison, other factions will recruit new hoplite squads to boost defence, sparta being the only faction to recruit catapults as far as I've seen.

I recently saw a Greek city with a catapult unit, it wasn't Sparta though (one of the Tyrants' cities I think) -- this was during the Philip campaign and was a nice surprise since I'd read they don't recruit catapults.

One of the things I'd like to see in general is more unit variety, especially among the Greek cities which don't have the same access to the 'barbarian' units that Macedon does. The Sandbox campaign seems a bit desolate with such limited unit selection right now...

Well thats special O.o

Yeah ive seen the Corinthian league and a few other city states around the Peloponnese with catapults. Ive even had the Phoecians use it against one of my cities when playing as the Boeotian league.