Beds can respawn you into dangerous blocks, creating death loops. The fact that you cannot respawn into solid blocks shows this is not intended. Mobs don't spawn into or on dangerous blocks.
You can use lava, you can even use water, breaking obsidian is too slow, you drown before breaking anything. You cannot respawn in air, but player detectors exist and can be used to retract the block and drop the player to death. This could of course potentially happen accidentally. Scenario: Bed in a corner of an obsidian room dug out of a lava lake, behind the corner is lava, it's first priority or the other spots are blocked by blocks like chests.

How to fix: Add a "Respawn at world spawn" button. Preventing respawn inside of lava blocks would be nice too, but that alone wouldn't fix all death loops and blocking respawn in water is not a good idea since the update aquatic.

One further comment on this: These death loops are known as "bed ban", which should clearly show the issue, non-op players can ban other players, which is something insane to have to worry about.