Buttons cannot be Shift-clicked onto hoppers or placed on other non-solid blocks (upside-down stairs and half-slabs, etc.). I believe this to be a bug because every other major redstone component can be placed on them (redstone dust, repeaters, torches, comparators, pressure plates, etc. ), and because buttons were recently allowed to be placed on the ground, it is possible they forgot to allow them to be placed on non-solid blocks.