i found the problem. the useraccount i used for the import_logs.py had only view rights. i gave it admin rights and now the data appears in piwik.
this should be changed in the import_logs.py output: the api-call requested a user with view rights and did not return an error on reading the logs and trying to import them to the db. it should request a user with admin rights and/or return an error if the data is not saved to the db.