I can replicate the same issue on my end and we did open a JIRA as well. It seems in an earlier version of AEM 5.x, 6.0 and 6.1, it was working as a side effect.
The intent was to never allow drag and drop functionality through siteadmin. Just for your records, the internal reference for communication with Engineering was CQ-4197704.
This functionality is no longer available in the product.
Regards,
Vishu