conversion operator as standalone function

The one reason I can think of is to prevent implicit conversions being applied to the thing being cast. In your example, if you said:

 bool( "foo" );

then "foo" would be implicitly converted to a string, which would then have the explicit bool conversion you provided applied to it.

This is not possible if the bool operator is a member function, as implicit conversions are not applied to *this. This greatly reduces the possibilities for ambiguity - ambiguities normally being seen as a "bad thing".