Guys, Ive question regarding the Entry Credits. Companies dont have to touch Factoids at all to make entries in the Factom System, which by the way is brilliant beacuse it reduces the risk of holding a cryptocurrency. However, I dont understand why Entry Credits cant be stolen. In the whitepaper is stated following:

Entry Credits are non transferable, so the user cannot assign them to another user’s public key, and selling private keys isn’t practical or useful.

Why is it not possible to transfer a entry credit?

Thx in advance!