I might get downvoted for that, I still gonna say it: That might be due to the demand-supply situation in photography. They probably will find photographers doing this for the promotion effect described in the email.
The question is, will they get multiple people on the shoot for free, and will that get them better pictures than one paid pro?
Can you imagine doing the work for free only to find out you're competing with other photographers on the same shoot to get the best shots that they will want to publish so you can get your "exposure payment".