Okay, if that is really the consensus then I can’t do much about it. I just thought that the Sulfur Cube with an absorbed block are neither “mob-like” (spawn eggs) nor “block-like” (solid blocks / water) and so are closer to boats, minecarts, etc. So I kinda expected that they would naturally be placeable in Adventure Mode.
And also, this might be the best way to go because one can simply use a pair of Shears to remove the absorbed block, virtually allowing the player to spawn a Sulfur Cube.