Pistons cannot be mined faster using a pickaxe than using bare hands, even though their material is stone. This is not consistent with other stone products: e.g. cobble walls are also mined faster using pickaxes.
Same applies to glass, beacons, conduits, sponges, sea lanterns, redstone lamps, glowstone, hay bales, cake, turtle eggs, cacti, and beds (all tested).
(I would expect to mine beds and cacti faster with an axe, to mine glass, beacons, conduits, sea lanterns, redstone lamps, and glowstone faster with a pickaxe, and to mine hay bales, kelp blocks, and sponges faster with shears. Most of these "features" are intentional for no good reason, but cake and turtle eggs may not need a tool to mine them since there is basically no reason to do so, unless you want to grief, or collect turtle eggs with silk touch. See MC-11992)
The piston is crafted out of cobblestone, its texture is majorly cobblestone, and its breaking and placing sounds are stone sounds; thus suggesting that it can be broken faster with a pickaxe. Yes, it's been like that since Beta 1.7.3, but that doesn't necessarily mean that it's not a bug.
Same applies to glass, beacons, conduits, sponges, sea lanterns, redstone lamps, glowstone, hay bales, cake, turtle eggs, cacti, and beds (all tested).
(I would expect to mine beds and cacti faster with an axe, to mine glass, beacons, conduits, sea lanterns, redstone lamps, and glowstone faster with a pickaxe, and to mine hay bales, kelp blocks, and sponges faster with shears. Most of these "features" are intentional for no good reason, but cake and turtle eggs may not need a tool to mine them since there is basically no reason to do so, unless you want to grief, or collect turtle eggs with silk touch. See MC-11992)