I’d run a mile from this. Personally, I would find it unacceptable as a guest. That is, if I knew about it. Would guests know?
Would your management company use the email addresses exclusively to contact your previous guests to stay with you again or would they use them for other marketing for other hosts / businesses too? More than likely the latter.
You have no control over their usage of that data. Who knows what they will do with those email addresses? I can’t imagine a scenario that will benefit you exclusively.
I would be very wary of this because email marketing worked fairly well at the beginning of this century but not these days.
Remember with your management company that you are the client. They work for you and you’re the boss. If you don’t agree to the data capture simply tell them so.
By the way, the best way to capture email addresses is simply to have a guest book with a column marked ‘email address’. Then the guest has the choice. But that is information for you and not the management company.