Why are all of the test cases added automatically every time i create a parent test filter? Also, deleting tests added in that filter deletes them for all the filters under which they are available?
Filters in Practitest show the entities which are defined by their Filter Criteria.
The entities themselves are not located in the Filters (unlike Folders) - Filters simply represent them.
For more information regarding Filters please refer to this article: How To Use Custom Filters - PractiTest