Problem occurs when there is a large queue behind a check-in desk made up of pax that arrived before the 30 minute cut-off. Check-in staff won't turn them back - if they did, there'd be a riot.
I've arrived many times at CDG (I'm cabin crew), finished the turnaround tasks as fast as possible only to find out from the despatcher that, with 10 minutes to go before departure, the check-in still hasn't closed.