... or have been so illegal. As you'll see in this article, pretty much all the cool shit you can do in video games breaks some kind of international law.
The taking of hostages -- even if done for reasons like "not dying" -- violates both the International Convention Against the Taking of Hostages (duh) and the Geneva Conventions. In both, the main argument is that by incapacitating an enemy enough to be able to take him hostage, you've rendered him harmless (or "hors de combat"), yet are still forcing him to participate in hostilities against his will. If you're gonna do that, then you might as well send soldiers into combat with kittens attached to their body armor, and that's a slippery slope we don't want to go down.
"But wait," you might say in the comments section, "both of those examples are from games in which you play as black ops, so they can't be charged with war crimes!" Firstly, in Black Ops, your character is part of MAC-V SOG, a special operations unit created by the government to fuck up the Vietcong, making him (legally speaking) a fully-fledged combatant. Secondly, in Splinter Cell, you work for the NSA, again making you a member of the armed forces involved in combat. Thirdly, *fart noise*. Hope that clears things up.