In short, yes - this is the default behavior with tags.
Behind the scenes, there are three tables involved when tagging a document:
- CMS_Document
- CMS_DocumentTag
- CMS_Tag
Whilst the tag itself can exist, the associate with the document cannot until it has been created and has an ID as you suggest. If I have to guess, I'd say that there is the potential to do some cusotmization here with a alternative form for creating your page types and a global event handler to fire after the doucment has been created.