I can't say I exactly have fond memories of ammunition management in BG. Nor do I remember ever running out of basic ammunition, so it seems a *shrug* for me unless there is some storage limitation that would make deciding how much ammunition to carry an important choice.
If you want to rationalize base ammunition being inexhaustible, assume everyone has a "create ammo" feat they use in camp. I'm sure mods will come to the rescue, if it bothers anyone enough to write a mod for it.
