I think you're one of the lucky ones, then, that the anatomy of your ear canal allows the earplug to seal the passage without the pillow putting pressure on it.

What almost works for me is using two differently sized earplugs and putting the smaller one in the pillow-side ear, because the pillow pushing on the ear canal causes it to seal despite the smaller plug. But when I roll onto my other side several times a night, I'd have to wake up and swap ear plugs. It's not feasible. :(