Doors are made of meshes rather than simply voxel blocks, so they don’t have the same restrictions as normal blocks. We use some fancy code magic to determine which parts of the door mesh to use where, depending on how you’ve placed the pieces.
Using meshes instead of voxels is one possible solution for glass as it sidesteps the alpha transparency issue(meshes can have semi-transparent textures as you may have seen on some items), but they’re not as flexible as voxel blocks and we don’t want people to be building whole structures out of meshes as that could lead to problems.