I don't think this is really question about scam in legalistic accounting sort of type. But it could very well count as moral scam.
As far as I understand the circuit or district will actually never see the true costs of assembly hall, they just buy services (facilities for convention) from it or in question about 3rd party facilities they are kind of sub-renter of organisation. Again without knowing what is the true cost. As any good organisation or firm selling services assembly hall/organisation will include certain amount of profit in their offer. Offer seems to be based on attendant fee. In this case circuit/district never takes competing offers and always accepts what is proposed. So before the convention starts circuit/district has agreed to pay certain amount of money (pre-deficit), and organisation is already at this point due for profit from forthcoming convention. I guess they will also calculate some sort of fee for released litterature too.
At conveniently enough stage of convention the financial status for this specific convention (not for assembly hall in general or 3rd party facility) is announced, donations in convention are calculated as circuits/districts income and not yet as part of the fund for worldwide work. If in ideal case the story at this stage shows that convention costs (again not true costs but the offered attendee fees) are not yet met, this should lead to financial activity around donation boxes, as for sure friends do not want to leave their circuit in deficit.
If everything works as expected, the donations will cover the conventions costs (remember not the actual costs but the offered rate), and the surplus can then be donated to organisation for worldwide work. So, what is the income of the organisation, firstly the profit calculated in attendee fee compared to true costs of running the assembly hall or 3rd party facility. On top of that they will receive the surplus donation.
What if everything was based on open books principle without profit margin? Well the convention cost would drop for circuits/districts. It could in worst case mean that the donations would in very early stage cover for the true costs, and announcing to public that the convention is already in surplus would at least in some cases keep the wallets closed, "ok, convention got clear papers, I can then donate back home to boxes in Kingdom Hall, it's much simpler that way". Do you think some of potential donors would not remember this "promise" when they got back home?
This is not scam for sure, this is exactly how business-world works, every value adding actor wants/needs to add their profit in calculation. What might be morally scam is that this pre-defined profit is not openly declared, I think most of the people there thinks that announced cost must be about the true cost based on experience from earlier conventions.
I do not know if my speculation is worth a wooden penny but this is my understanding of the concept here. This is not the only case where profit is present in hidden form, it was there already when the books and mags were sold.
CP