Subtasks in task templates can be added to projects, it’s just a little hidden, consistent with how it’s hidden for regular (non task-template) subtasks. You have to ask for the field to be shown either with Tab+P or the menu:
Or am I missing something?
I was never a fan of this design, but perhaps Asana found that helped the majority of users in user testing (perhaps avoiding mistakenly adding subtasks to projects when that wasn’t really what they meant to do).
Thanks,
Larry