That because it doesn’t really make sense to have activities regarding a subject.
An activity only really makes sense in the context of something else, and that something else is generally a real thing in the world or a piece of work. The Regarding field really says: "This Activity involves working on this thing".
For example, if John Smith rings your company, it makes sense to create a new Phone Call activity that Regards the Contact record of John Smith. With this approach you can see the entire activity history against a Contact. By making the Phone Call regard the Contact you are saying this piece of work is about the Contact.
If the Phone Call was Regarding a Subject, it would be saying, "This Phone Call involves working on the Subject", which isn’t true, its work about the Contact. Also you lose the ability to track correspondence with the contact.
So my suggestion would be, use the Regarding field to link to people and things, and add a relationship between your activities and subject if you want to classify your phone calls.
As a side, there is a product entity in CRM (it can’t be Regarding however), so perhaps you should create a relationship to that.