That would be because you're telling the code to strike out all of them.
A better option would be to add thisTodoStriked
to the item.
I don't know CoffeeScript, but the equivalent JavaScript:
$scope.delete = function (item){
$scope.todoItems[item].thisTodoStriked = true;
}
Then, in your template:
<span class="todo-title"
ng-class="{striked: todo.thisTodoStriked === true}"
>{{todo.title}}</span>