I think the math goes thusly,
Capturing units in combat is difficult (megalogwiffs aside). Captured units are difficult to turn and suffer from low loyalty. Thus, only really valuable units are captured and tried to turn in combat.
However, a city's garrison captured means all units still in the city are automatically shackled. So, hey! Captured units, what to do with them.
Turning them is still probably not a good idea. That would seem to leave execution as the only alternative...
Or does it? Historically in warfare, its standard operating procedure to offer captured units back to their side in exchange for an appropriate ransom. This ransom is typically either money or similarly captured units.
The last option being common, or at least not unheard of, in Erf would not surprise me in the least.
However, in Book 1 this was not an option. GK was the last GK city, when it fell there was going to be no one to ransom captured units to. Furthermore, Stanley and eventually Parson's behavior (such as the mass decryption of Jetstone's finest) made Ansom very unwilling to cut the side any slack. Yet, Charlie got Parson's capture out of Ansome regardless.



