Because that system wide setting would break a ton of apps (think about games that only use landscape for example). I mean, I'd personally be fine with this, but I don't think it's as easy as that.
Rotation is a system-provided feature. Apps that require the accelerometer access it directly; apps that require landscape would still be able to force that orientation (and would work as they do today)
I'm using Rotation Inhibitor on a jailbroken 3GS and it doesn't break anything at all.
Be sure to thank boucher for implementing this in the reddit app.