Sending "subscribed" presence to a new user will update roster of both users
Section 8.2.8 of rfc3921 says:
"Upon receiving the presence stanza of type "subscribed" addressed to the user, the user's server MUST first verify that the contact is in the user's roster with either of the following states: (a) subscription='none' and ask='subscribe' or (b) subscription='from' and ask='subscribe'. If the contact is not in the user's roster with either of those states, the user's server MUST silently ignore the presence stanza of type "subscribed" (i.e., it MUST NOT route it to the user, modify the user's roster, or generate a roster push to the user's available resources)."
Wildfire is not ignoring subscribed presences sent to users that are not in the roster.