Returning ElementCriteriaModel of entries via plugin
Of course this is the expected behavior because you don't return an ElementCriteriaModel from your plugin.

You do return $criteria-&gt;find(); so you'll return an array of entries but not an ElementCriteriaModel if you want to return the object instead of the entries you have to do return $criteria.

As soon as you use the find() method Craft builds a query based on your ElementCriteriaModel, executes it and returns you an array of elements or null.