When you request a script that has a .head.ftl part, the framework will ensure the resources are in the <head> section of the response.
However, for an AJAX request (such as from the SimpleDialog module), browsers don't interpret the response as a "normal" page load and have to be told what to do. Currently, we support <script> blocks within AJAX responses (e.g. for setting i18n messages, etc.) but not dynamic loading of .js and .css files. It's on my list to add, but because it's a non-trivial problem to solve, it's not been prioritised yet.
It's the reason why, for example, the documentlist.get.head.ftl file contains the .js and .css for all the actions UIs, such as assign workflow, copy to, etc.
Thanks,
Mike