Why AngularJS currency filter formats negative numbers with parenthesis?

Question

Live Demo

Why this:

# Controller
$scope.price = -10;

# View
{{ price | currency }}

results in ($10.00) rather than

Answer 1

Update: Angular 1.4 no longer uses parentheses to indicate negative values but now uses the "-" symbol. Here is a link to a discussion: https://github.com/angular/angular.js/issues/12870

I used the decorator as described by marc to return the .negPre and .negSuf to use the parens.