I got the email below from Alamy today. This only seems like a positive, but as a result I�m not sure why they�re asking rather than just demanding. Am I missing something?

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Would you like to increase your revenue from Collecting Society payments?

Alamy currently works with several Collecting Societies on your behalf to claim royalty payments on secondary uses of your images. We pay you 50% of commission of the funds awarded. You may have already received payments related to the work we do with ASCRL and other Collecting Societies. 

We now have the opportunity to apply for a greater share of the royalty payments, which you can benefit from by sharing your date of birth with us. This is a new administrative requirement of some of the funders, including ASCRL, who want this additional information so they can identify copyright owners more easily.

I got the same email and have the same question.

I received the letter too and gave my date to Alamy, even if the request is ridiculous, as they have all my data anyway.
If you follow the link in the email ("If you need further guidance, read more about how we work with Collecting Societies...")  you get your explanation: it is a secondary copyright income from no profit societies.
I'm in this scheme already, and every year I receive extra income, not a lot but nice 

I've got the email
but dont see anything on alamy site or their forum
will wait

Alamy confirmed on the forum that this is genuine several days ago. Pretty sure I didn't have to give my DOB when I signed up so they don't have that information already ( how this helps collection agencies track uses I have no idea). Completed and submitted!