my guess: to not complicate parsing complex expressions in default parameters

  // non-strict mode top-level code
  
  function foo (complex = eval("1+1"), moreComplex = (function(){ with (this) { return bar; } }())) {
      
	  "use strict"; // go back and parse the parameters again but this time with strict mode semantics
      
	  // ...
	  
  }