Squirrels are tougher than gophers. Because gophers almost entirely live and die underground, more poison sre (or at least were*) available for gophers. Because squirrels have a higher probability of dying above ground there is more concern about secondary kill. As a result poisons legal to use on squirrels are more limited.

*The Wilco 1.8% strychnine gopher bait used to be the bomb. Then it wasn’t available, but the 0.5% was, though hard to get. When I asked why at the Farm Supply store, they said DHS had clamped down on the import or strychnine and Wilco was making only the 0.5% bait as they could make and sell a lot more of it with the limited amount of strychnine they could import. I dunno that even the 0.5% is available anymore. Neither could be used legally on squirrels due to concerns about a secondary kill with strychnine.

Mice, squirrels, rats, gophers, prairie dogs, woodchucks, marmots, rockchucks, chipmunks, groundhogs, voles -- they're all rodents in my book. Pests. They eat things that humans don't want eaten. They dig where it's a pain for humans and livestock. I don't get why any human on this planet thinks they're cute. I realize that they have a place in the overall order of nature, but I consider them little more than pests. I put a round of .223 Remy into the chest of a PD a few weeks ago -- good riddance.

When we lived in the high desert of central Oregon, had a ton of ground squirrels and a few other little rodents on our property. Added a RWS air rifle in.177.. holy crap was that fun. Scope, muzzle brake (more for show) and at 1000fps, whittled that population down in no time.