Why is using the function name as a function pointer equivalent to applying the address-of operator to the function name?

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Since you specifically ask for the rationale of this behavior, here’s the closest thing I can find (from the ANSI C90 Rationale document – http://www.lysator.liu.se/c/rat/c3.html#3-3-2-2):

3.3.2.2 Function calls

Pointers to functions may be used either as (*pf)() or as pf().
The latter construct, not sanctioned in the Base Document, appears in
some present versions of C, is unambiguous, invalidates no old code,
and can be an important shorthand. The shorthand is useful for
packages that present only one external name, which designates a
structure full of pointers to object s and functions : member
functions can be called as graphics.open(file) instead of
(*graphics.open)(file). The treatment of function designators can
lead to some curious , but valid , syntactic forms . Given the
declarations : 

int f ( ) , ( *pf ) ( ) ;

then all of the following expressions are valid function calls :

( &f)(); f(); (*f)(); (**f)(); (***f)();
pf(); (*pf)(); (**pf)(); (***pf)();

The first expression on each line was discussed in the previous
paragraph . The second is conventional usage . All subsequent
expressions take advantage of the implicit conversion of a function
designator to a pointer value , in nearly all expression contexts .
The Committee saw no real harm in allowing these forms ; outlawing
forms like (*f)(), while still permitting *a (for int a[]),
simply seemed more trouble than it was worth .

Basically, the equivalence between function designators and function pointers was added to make using function pointers a little more convenient.

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