Several areas of Satchmo compare a datetime from the database with datetime.now() which results in an exception because timezone aware and timezone naive datetimes cannot be compared.
To reproduce one such instance, set USE_TZ to True and visit a download link. The call to DownloadLink.is_valid() will generate such an exception.
Since Satchmo supports Django 1.4 and is looking to support 1.5, using Django's timezone.now() is a simple way to address this issue. There is no need to worry about it not being available in lower versions since they are no longer supported.
At the moment, Django projects that want to use Satchmo are not able to set USE_TZ to True and remain functioning.