Well, the purpose of subscriptions in the context of CKAN is to produce datasets for personalized activity streams or for displaying these datasets that adhere to the definition of the subscription.
However, there might be other objects that could adhere to a subscription definition that aren't datasets.
There are three categories of object:
1) datasets
2) domain_objects other than datasets (cf. extension)
3) other objects that have no domain objects in CKAN (cf. extension)

Maybe, it would be better to have an action where the caller can specify which object type he wants.