I find it to be exactly the opposite. Django Tastypie is in no way tied to the Django Orm.
I'm looking at ModelResource as my example.
If you aren't working with a NoSQL database or a search engine, or grabbing data from an tertiary resource then the sheer complexity found in TastyPie might not be for you.
Django-Piston doesn't offer many features, but its dead simple in its usage.