As to the second query, it is the law. Note the last sentence. A President would have had to sign this into law, just like any other federal law (OK I know that there are other ways like overriding a veto, etc.) I’m too lazy right now to research which prez did.
United States Code
TITLE 18 - CRIMES AND CRIMINAL PROCEDURE
PART II - CRIMINAL PROCEDURE
CHAPTER 203 - ARREST AND COMMITMENT
U.S. Code as of: 01/05/99
Section 3056. Powers, authorities, and duties of United States Secret Service
(a) Under the direction of the Secretary of the Treasury, the
United States Secret Service is authorized to protect the following
(1) The President, the Vice President (or other officer next in
the order of succession to the Office of President), the
President-elect, and the Vice President-elect.
(2) The immediate families of those individuals listed in
(3) Former Presidents and their spouses for their lifetimes,
except that protection of a spouse shall terminate in the event
of remarriage unless the former President did not serve as
President prior to January 1, 1997, in which case, former
Presidents and their spouses for a period of not more than ten
years from the date a former President leaves office, except that
(A) protection of a spouse shall terminate in the event of
remarriage or the divorce from, or death of a former President;
(B) should the death of a President occur while in office or
within one year after leaving office, the spouse shall receive
protection for one year from the time of such death:
Provided, That the Secretary of the Treasury shall have the
authority to direct the Secret Service to provide temporary
protection for any of these individuals at any time if the
Secretary of the Treasury or designee determines that information
or conditions warrant such protection.
(4) Children of a former President who are under 16 years of
age for a period not to exceed ten years or upon the child
becoming 16 years of age, whichever comes first.
(5) Visiting heads of foreign states or foreign governments.
(6) Other distinguished foreign visitors to the United States
and official representatives of the United States performing
special missions abroad when the President directs that such
protection be provided.
(7) Major Presidential and Vice Presidential candidates and,
within 120 days of the general Presidential election, the spouses
of such candidates. As used in this paragraph, the term ''major
Presidential and Vice Presidential candidates'' means those
individuals identified as such by the Secretary of the Treasury
after consultation with an advisory committee consisting of the
Speaker of the House of Representatives, the minority leader of
the House of Representatives, the majority and minority leaders
of the Senate, and one additional member selected by the other
members of the committee.
The protection authorized in paragraphs (2) through (7) may be