Broken Magenta Line on Sectional "National Security"

About 35 miles southwest of KLSC, Salt Lake City Intl (just beyond the
Mode C veil) there is a box marked by a thick broken magenta line on the
Salt Lake sectional. Nearby, there is a Notice: "For National Security
reasons, pilots are requested to avoid flight at/below 4,000' in this
area."

The area is around one of the chemical weapons stockpiles and
incinerator facilities in the vast Dugway Proving Ground area. Is this
broken magenta line used anywhere else? Does it have an official
meaning?

If National Security is the issue, why isn't the area marked as
Restricted instead? Perhaps it should say "For reasons of your health
and not breathing toxic chemical weapon waste smoke, you might want to
fly elsewhere?