One reason for this is economy (in the patients favor, for once). Controlled pain meds require periodic visits to check up/monitor. Office visits to PCPs cost less/have lower co-pays than to specialists, and specialists often "cut patients loose" if there's nothing further they can do for them (other than writing more scrips). PCPs also act as coordinators between specialists, so having them write pain scrips is a way of avoiding duplications/doctor shopping.

Doc