More details here: https://premium.wpmudev.org/forums/topic/disable-calendar-constraints
Basically, since is already possible to type dates in the past, it would be more usable to disable the constraint that doesn’t allow to use a past date with the date picker.
People must be free to publish old events, especially when they’re migrating an old site to a new one and wants to recover as much data as possible.
And example: a band that wants to keep an archive of their concerts.
Right now the behavior is confusing and quite user “unfriendly”.