We prefer to view this as offering an incentive for people to show up on time by offering an “early-bird” discount.
The sessions are quite information-dense and require full attention of participants from the very start of the presentations. Showing up part way through a session means that you miss out on important background or introductory content that gives context to the rest of the session. Moreover, it disrupts the talk in progress affecting both the speaker and the attendees.
We totally understand that some of you absolutely can’t come on time and that you have legitimate reasons. That’s the only reason why we don’t lock the door completely.
If this policy does not achieve our goal of getting people to the event on time, we may yet resort to locking the door or teleporting the meeting to an undisclosed location on another planet.