The advancement "Free the End" is supposed to be a reward for the player's endeavor to kill the ender dragon. Since the dragon is unlikely to take damage from natural causes, if the dragon dies close to the player, even if the player does not deal the last blow to it (for example, by exploding a bed), it is still evident that the player has actively engaged in killing the dragon. For a boss mob like the ender dragon, the player should be able to defeat it using any method possible, so it does not make sense not rewarding the advancement just because the dragon isn't killed directly by the player.
Another issue is on multiplayer servers. When several players enter the end and fight the ender dragon, they have all contributed to the fight, so it is natural to reward all players involved the advancement. However, currently the advancement is only given to the player that deals the last blow to the dragon, which can cause quarrels between the players over who deals the last blow. This kind of quarrel isn't constructive, therefore to encourage collaboration spirit between the players, the advancement should be awarded to all players involved in the fight.
One more reason: the advancement "The End... Again..." appears to be triggered by being in proximity to the exit portal when the dragon is re-summoned using end crystals. If this is done while multiple players are in the End, all players get the advancement. Therefore for consistency, "Free the End" should be triggered with a similar proximity-based criteria, as both advancements involve the ender dragon and the exit portal.

The solution to this problem is simple: Change it so that "Free the End" is granted to players within a certain radius from the exit portal when the ender dragon dies. The criteria can be made identical to that of "The End... Again...", or different depending on the developer's preferences.