Why is `throw` invalid in an ES6 arrow function?

If you don’t use a block ({}) as body of an arrow function, the body must be an expression:

ArrowFunction:
    ArrowParameters[no LineTerminator here] => ConciseBody

ConciseBody:
    [lookahead ≠ { ] AssignmentExpression
    { FunctionBody }

But throw is a statement, not an expression.

---

In theory

() => throw x;

is equivalent to

() => { return throw x; }

which would not be valid either.