From what I can tell, Beamdog has added content such as additional party members (e.g. wild mage) and an expansion (i.e. Siege of Dragonspear) to its games. You also have entire forum sections encouraging users to create mods, update existing ones that were previously created, and, presumably, to install those mods for their own enjoyment.
The Icewind Dale and Baldur's Gate games are from an era when user-created content wasn't fully appreciated. These days, games often have separate modding agreements spelling out what users are and aren't allowed to do. Skyrim's EULA, for example, allows its users to create mods: http://store.steampowered.com/eula/eula_202480
Note the provision "You are only permitted to distribute the New Materials, without charge (i.e., on a strictly non-commercial basis) (except as set forth in Section 5 below), ...". This provision explicitly allows the distribution of mods.
No matter the reasons for the lack of a modding agreement, the fact remains that it is, strictly speaking, against copyright law for users (in the absence of another agreement granting the requisite permissions) to create or install mods. Mods are clearly derivative works of the original game, and even installing a mod for personal enjoyment clearly doesn't fall under fair use.
Therefore, the only way Beamdog could legally create and allow the installation of mods would be if they had the necessary permissions from all the copyright holders. Without such permission, it would be against copyright law for even a store to allow their customers to install mods. For example, Gog.com is purely a distributor of games and does not have the required permissions. Thus, although they may tolerate posts in their forum on how to mod the game, it is against copyright law.
I believe Beamdog's legal situation might be different, but I would prefer to hear from the founders (or someone with knowledge of this matter) rather than relying on my pure speculation. Tolerating talk in one's forum and actually creating content without permission are quite different, and I strongly believe Beamdog would not do something that is against the law. The creation of extra characters in a game and the creation of an expansion pack are two clear examples of the creation of derivative works. Based on my assumption that Beamdog abides by the law, therefore, it seems reasonable to conclude that Beamdog does have all the necessary permissions to:
1. Create derivative works for the games they are selling (e.g. expansion packs, mods, extra characters)
2. Grant users permission to create derivative works (e.g. expansion packs, mods, extra characters)
Is my conclusion correct, and if so, when and from whom did Beamdog obtain the necessary permissions?