Autoscaling group doesn't delete scaled-up instances on stack delete

When you run the autoscaling example template, if you trigger a scaling event such that the scaling policy adds another instance, then delete the stack, the scaled-up instance is not deleted:

Steps to reproduce:

1 - Create Stack

heat create autoscale --template-file=templates/AutoScalingMultiAZSample.template --parameters="InstanceType=m1.xlarge;DBUsername=${USER};DBPassword=verybadpass;KeyName=${USER}_key"

2 - Trigger autoscaling event (either via on-instance activity, or via heat-watch as below)

heat-watch set-state MEMAlarmHigh ALARM

3 - Wait for scaling policy to build a new instance

4 - Delete the stack

5 - Observe the scaled-up instance is not deleted

[root@heatlt heat]# heat list
<ListStacksResponse>
  <ListStacksResult>
    <StackSummaries>
      <member>
        <StackId>arn:openstack:heat::9028c3aaab5a4d09bc0423875e36c40e:stacks/autoscale/10</StackId>
        <LastUpdatedTime>2012-11-05T15:58:28Z</LastUpdatedTime>
        <TemplateDescription>AWS CloudFormation Sample Template</TemplateDescription>
        <StackStatusReason>Stack successfully created</StackStatusReason>
        <CreationTime>2012-11-05T15:51:41Z</CreationTime>
        <StackName>autoscale</StackName>
        <StackStatus>CREATE_COMPLETE</StackStatus>
      </member>
    </StackSummaries>
  </ListStacksResult>
</ListStacksResponse>

[root@heatlt heat]# nova list
+--------------------------------------+-------------------------------------------+--------+-----------------------+
| ID | Name | Status | Networks |
+--------------------------------------+-------------------------------------------+--------+-----------------------+
| 204b42e9-97f9-49d7-9b81-2b3c3d1197e9 | autoscale.ElasticLoadBalancer.LB_instance | ACTIVE | demonet=192.168.250.3 |
| 1c2e832a-9a26-4dc1-9a0f-ed7eb3a6151e | autoscale.WebServerGroup-0 | ACTIVE | demonet=192.168.250.5 |
| aef31ba7-d9bc-4e42-a9bc-8255661aaaae | autoscale.WebServerGroup-1 | ACTIVE | demonet=192.168.250.6 |
+--------------------------------------+-------------------------------------------+--------+-----------------------+
[root@heatlt heat]# heat delete autoscale
<DeleteStackResponse>
  <DeleteStackResult/>
</DeleteStackResponse>

[root@heatlt heat]# heat list
<ListStacksResponse>
  <ListStacksResult>
    <StackSummaries/>
  </ListStacksResult>
</ListStacksResponse>

[root@heatlt heat]# nova list
+--------------------------------------+----------------------------+--------+-----------------------+
| ID | Name | Status | Networks |
+--------------------------------------+----------------------------+--------+-----------------------+
| aef31ba7-d9bc-4e42-a9bc-8255661aaaae | autoscale.WebServerGroup-1 | ACTIVE | demonet=192.168.250.6 |
+--------------------------------------+----------------------------+--------+-----------------------+