Where there are pine cone middens there are squirrels. Figuratively speaking, squirrels and pine trees have been engaged in an evolutionary tango for a very long time. (Read a bit more here.)
The successful pine tree fends off chisel-toothed rodents that attack and eat its seeds before they even germinate. That doesn't mean it bars the wascally squirrels from eating its entire genetic investment.
It's more of a numbers game.
The tree makes the squirrel work harder for a smaller meal by putting fewer seeds in its cones and arming the cone's scales with painful spines and hooks.
Have you ever tried to remove the nuts from a pine cone using only your hands and teeth?
You might succeed with some of the smaller cones like those of Ponderosa pines, but it wouldn't be much fun and it certainly wouldn't be very rewarding, because the seeds are small.
So tear into a cone with large seeds, like those hooked bombs from our gray (or ghost) pines.
Your lips and cheeks would look like taco meat in no time. If you think that handsome beard would protect you, try removing the pitch without the benefit of solvents.
Squirrels have responded to the challenge by evolving larger heads and powerful jaws as well as effective ways or munching around those hooks and spines.
The Western gray squirrels here in the Sierra Nevada foothills are hefty bruisers. In addition to acorns they successfully tackle the nasty hooked cones of our gray pines. I doubt that a smaller squirrel could handle them very well.
But these big squirrels are built to take on big cones.
Steele, M.A. Evolutionary interactions between tree squirrels and trees: a review and synthesis. Special Section: Arboreal Squirrels (pdf)