Wouldnt it be better to be able to join sessions that are still in qualifying? Sometimes I hop online and see nice full races that I just missed and are still in qualifying.
Whether or not thats possible can be set by whoever runs the server. If youre asking about the ranked servers the reason that joining in quali is not possible is because it can often cause a little lag for those already on the server. In practice youll have to just live with that but if it ruins your quali laps thats rather frustrating and so the decision has been made to not allow joining at that point (dont know if thats final tho as it all still is in beta)