Doug McCrae
Legend
You're absolutely right from a what-makes-sense-in-the-game-world perspective.If you want surrender to be a normal option, you probably have to set is as a cultural norm within the lands the PCs inhabit.
I think there's a couple of perspectives where a lopsided approach - PCs never take prisoners but can be captured themselves - does make sense:
1) Genre convention. The protagonists in adventure fiction are very frequently captured but rarely take prisoners. Some examples from Appendix N: The Hobbit (dwarves captured by goblins), The Two Towers (Merry and Pippin by orcs), The Roaring Trumpet (Harold Shea imprisoned in the fire giant stronghold with trolls as his jailers), Tarzan at the Earth's Core (Jason Gridley imprisoned by lizard-men).
2) Player convenience. The prison escape can work as a form of adventure that's a bit different to the usual, but as you say problems can arise when the PCs have to deal with captives.