From what I can see in XMPP-IM section 7.1, any valid
JID can be a part of the Roster.
I couldn’'t find any text that specifies that any valid JID can be part of a roster. However, I found this text in section 7.1 which says: “The “key” or unique identifier for each roster item is a JID, encapsulated in the ‘‘jid’’ attribute of the element (which is REQUIRED). The value of the ‘‘jid’’ attribute SHOULD be of the form user@domain if the item is associated with another (human) instant messaging user.”
Unfortunately, the spec does not specify the valid form of a JID for items not associated with (human) instant messaging user. Although, we can say, in some way, that a gateway is some kind of IM user.
Could you provide me the exact text where you read/understood your statement?