I never got the impression that Giles was being presumptuous or unreasonable in that scene. We don’t get to hear what he actually said, all we do hear is Angel presumptuously ordering him to get Willow to deal with a colleague who’d been killed as a direct result of Angel signing up with Wolfram and Hart. We don’t know exactly what Angel said but under the circumstances he must be asking Willow to either resurrect a dead person (an act Giles described as killing everyone and risking hell on earth the last time she did it) or forcing Fred’s soul to share a body with an Old One. As stormwreath said Angel wouldn’t have been able to tell Buffy the real reason he joined W&H and probably came across pretty shifty if he attempted to do so. Under Angel’s CEOship W&H have continued to be involved with human sacrificing clients and profiting from the powerful, demonic and evil. Giles not wanting to send Willow to Evil Incorporated on some crazy half-baked and possibly hell-on-earth risking mission seems pretty reasonable.