My experience differs from your in that case. Normally as soon as the forward cabin is away they allow other cabins to exit as well. There's absolutely no reason for crew to hold you back for no reason.
Perhaps part of the reason for the curtain, rather than just a crew member, is that waiting folks get impatient? If they only see one or two stragglers in business class they may decide they want to go and pressure the crew member to stand aside. Maybe the rest of the business cabin is well inside the jet-bridge, but a business class passenger needed time to transfer to a wheelchair?
Going back to your earlier post - 'leaving economy passengers in the 'dark' when standing behind the curtain - I guess the airline figures passengers will know what's going on? That is, that the forward cabins are being cleared. What else do passengers think it could be? As I said, there's zero motive to be vindictive to waiting passengers. There's a reason why passengers would be kept waiting for a few seconds. But not just for the fun it.