It's just occurred to me that the free fares on New Year's Eve aren't solely a case of TfL (or their sponsors') benevolence, but more to do with the difficulty of correctly charging for journeys which span the 'start of ticketing day' at 0430.

https://nighttube.london/customer-information/ says that the ticketing system wasn't previously capable of this, but Cubic have recently updated the backend so that it can cope when (if?) night tube eventually starts.

I wonder if it's coincidence that free travel on New Year's Eve began on 31/12/03 - the first year that Oyster cards were available?