Was working fine in pre5, but broke in this snapshot, preventing a lot of creative freedom. According to the comments of MC-129127, placing blocks this way probably shouldn't be considered a bug.

To reproduce fly away from all blocks and run the command /setblock ~ ~ ~ torch - the command fails whereas it previosuly never would have.

Even if this is considered intentional I'd strongly recommend reverting to the prior behaviour, as this current change severely hampers creative liberty for mapmakers and Creative mode builders, who may want to utilise "invalid" states for their own purposes (e.g. the aforementioned floating torches, which are still completely stable until a block is placed below them and then removed), as well as making it quite difficult to demonstrate several bugs regarding blocks not being able to be placed in certain positions such as MC-174479, MC-147248 and MC-147249.