if i recall, Shutterstock you have to send an invite to gain that referral. dreamstime, i think the new member has just to give your id#. lastly, the latter referral is not forever, it is limited to 3 years.
and if your referral becomes exclusive later on, i think they end your referral. at least that's what happened to all my referrals.
like most sites, there will always be one form and another to cap what they pay you ie as soon as they don't need you, they throw you out like one used to do in the dark ages with the piss collected overnight the spittoon before shouting to the passersby "regarde leau"