Users cannot schedule activities on models for those they have no write permission.
However, they can schedule activities for themselves.

E.g.: User 1 belongs to group "See Subscriptions".
- He schedules an activity for himself on a subscription --> OK
- He schedules an activity for another user on a subscription --> NOT OK --> He gets the message: 

You are not allowed to modify 'Subscription' (sale.subscription) records.
This operation is allowed for the following groups:
- Subscriptions/Administrator
- User types/Portal"

Is there any way to avoid this, so that users who do not have write access to some model can program activities to other users on that model?