where there's trees there will be tree wells, where there's steeps and pow there will be avalanches.
in the backcountry you're skiing in unpatrolled or isolated areas which makes you harder to find and rescue, whereas in the park, you're probably directly under a lift.
you could die quickly in the backcountry even if you're skiing with a partner.
as for spinal or head injuries in the park, you can just as easily get a spinal injury by skiing backcountry.
"I'm in Manitoba, AKA the Canadian prairies. Cliffs and deep powder is like finding a unicorn." -AndrewCinch