If you purchased the premium version of Battlefield 1 you would have gotten access to all the DLC that has released thus far. The Fire and Ice Operations campaigns are very popular but it looks like DICE are still unhappy with the number of players they are able to get into a game. To make sure the online community continues to grow they are making the Operations DLC free for all as of today. DICE have also made all the previous Operations DLC free to play as well.
It looks like DICE may have realised that the paywall blocking the Operations maps led to a small online player base and hopefully this means the next Battlefield title will drop the Premium Pass. That said, the maps will only be accessible in the Operations mode and players will not have the weapons, vehicles, or Elites which exist in the corresponding expansions and maps.