Why use signed integers?

I've noticed that, in quite a few MMORPGs, the stackable item/money cap is (2^31)-1, or 2,147,483,647.

But why use a signed integer? Do you really need negative amounts of items?

Why not use unsigned integers and then have the cap be (2^32)-1?

>>23
Just because a server uses a 64-bit CPU and OS doesn't mean its users do. Many people still use old CPUs with 32-bit bus widths, and thus still use 32-bit OSes and require 32-bit game clients.