Did you forget that the tail wraps around your neck, and when you start prying on the legs they tighten their tail? They'll choke you to death if you try to remove them. You can't cut them off, because the powerful acid will kill the person. They're perfectly evolved for this one thing. Kinda crazy, since this is the first time they've met people, but they're already perfectly evolved to prey on people.
Don't underestimate inverts. Coconut crabs are a little bigger, and are strong enough to climb trees. An alien facehugger could be just as strong and you wouldn't be able to get it ofd with brute force.