Hi Marco
There is only one simple reason for that - SES Plans are generated against a PO. In other words, for processing them, you need only a PO with a service type line item. You do not use a work order for those.
If you want to use a strategy plan with SES, then 1) You need to use a work order generating category and 2) Your task list operations should have the control key with service on them.
Regards
Savas