How to tell Django collectstatic to use Amazon S3?

Question

I have followed this howto to setup django + S3. Specifically:

import os

# AWS credentials
AWS_STORAGE_BUCKET_NAME = os.environ['AWS_STORAGE_BUCKET_NAME']
AWS_S3_CUSTOM_DOMAIN = '%s.s3.amazonaws.com' % AWS_STORAGE_BUCKET_NAME
# boto config
AWS_ACCESS_KEY_ID = os.environ['AWS_ACCESS_KEY_ID']
AWS_SECRET_ACCESS_KEY = os.environ['AWS_SECRET_ACCESS_KEY']
AWS_HEADERS = {  # see http://developer.yahoo.com/performance/rules.html#expires
    'Expires': 'Thu, 31 Dec 2099 20:00:00 GMT',
    'Cache-Control': 'max-age=94608000',
}

# For the static files
STATICFILES_LOCATION = 'static'
STATICFILES_STORAGE = 'myapp.custom_storages.StaticStorage'
STATIC_URL = 'https://%s/%s/' % (AWS_S3_CUSTOM_DOMAIN, STATICFILES_LOCATION)

# For the media files
MEDIAFILES_LOCATION = 'media'
DEFAULT_FILE_STORAGE = 'myapp.custom_storages.MediaStorage'
MEDIA_URL = 'https://%s/%s/' % (AWS_S3_CUSTOM_DOMAIN, MEDIAFILES_LOCATION)

My custom storages are simple S3BotoStorages:

from django.conf import settings
from storages.backends.s3boto import S3BotoStorage


class StaticStorage(S3BotoStorage):
    location = settings.STATICFILES_LOCATION


class MediaStorage(S3BotoStorage):
    location = settings.MEDIAFILES_LOCATION

I would expect that collectstatic would honor this configuration (as explained in the howto), and use the myapp.custom_storages.StaticStorage to upload collected static files to S3. Instead, it just uses the local filesystem. Since I have:

STATIC_ROOT = os.path.join(BASE_DIR, 'mycollectstatic')

(just because having 'static' there seems too confusing to me), I can clearly see that:

» python manage.py collectstatic

You have requested to collect static files at the destination
location as specified in your settings:

    /absolute-path/mycollectstatic

This will overwrite existing files!
Are you sure you want to do this?

Type 'yes' to continue, or 'no' to cancel: 

So it seems that collectstatic command is using STATIC_ROOT even when STATICFILES_STORAGE = 'myapp.custom_storages.StaticStorage'. Is this expected?

Should I configure STATIC_ROOT differently when using another STATICFILES_STORAGE? Where is this documented?