Is anyone grounded enough in the tech/law here to explain why the app vendor doesn't funnel user data to FB from their backend? Connecting directly to FB seems

Is anyone grounded enough in the tech/law here to explain why the app vendor doesn't funnel user data to FB from their backend? Connecting directly to FB seems like an unnecessary giveaway. Is it just about ease of implementation or are there legal implications? People have quoted the wiretap act to me but the argument doesn't make sense.