Redstone ore can only be mined using an iron or diamond pickaxe, but redstone blocks can be mined with any pickaxe.
Linked issues
is duplicated by 2
Comments 3
MC-5 is a similar issue with emeralds, and it has been fixed, so I think this one is also a bug.
It has been this way as long as I can recall, the Wiki even mentions it in the trivia section of the "Block of Redstone" page.