They have been for most of my life. If they were all closed, it was a fairly recent event.
from
https://www.nps.gov/goga/planyourvisit/designations.htmNational preserves are areas having characteristics associated with national parks, but in which Congress has permitted continued public hunting, trapping, oil/gas exploration and extraction. Many existing national preserves, without sport hunting, would qualify for national park designation.
Maybe you're confusing therm with national parks.: National Park
These are generally large natural places having a wide variety of attributes, at times including significant historic assets. Hunting, mining and consumptive activities are not authorized.