The Proper Way for storing arbitrary commands or arguments in a variable would be to use an array, see How can we run a command stored in a variable?
But you can just tell shellcheck you like it just the way it is, as long as you're sure it's ok (i.e. you don't have whitespace that should stay intact, or glob characters that could cause issues, and didn't change IFS to something that would trash this):
AREA_ARG=""
if __SOME_SETTING__ ; then
AREA_ARG=" --area us,ca "
fi
# shellcheck disable=SC2086 # split on purpose
process_data -i /some/path $AREA_ARG