I would guess that it's just good programming practice to feed a function the context in which it would normally be called -- which for the Math.max function is the Math namespace object.

It probably makes no difference what context the Math functions are executed in, since they are probably backed by native code implementations that don't rely on context, but one could imagine some JavaScript implementation taking a shortcut and doing something like:

  Math.exp = function(p) {
    return this.pow(this.E, p);
  }

That function wouldn't work correctly unless it was executed in the Math context:

  assert(Math.exp(1), "Works");
  assert(Math.exp.apply(Math, [1]), "Works");
  var obj = {};
  try {
    Math.exp.apply(obj, [1]);
  } catch(e) {
    assert(false, "Doesn't work.");
  }