abstract class member functions can not have contracts

[ref2401]

Interface member functions can have in/out contracts but why 
abstract class member functions can not?

abstract class Base
{
	@property int field();

	void foo() in { assert(field > 0); }	
// Error: function main.Base.foo in and out contracts require 
function body
// An empty body definition fixes this error.

}

interface IBase
{
	@property int field();

	void foo() in { assert(field > 0); } // OK
}