Polearm Master (page 168, Player's Handbook) says
You can keep your enemies at bay with reach weapons. You gain the following benefits:
• When you take the Attack action and attack with only a glaive, halberd, or quarterstaff, you can use a bonus action to make a melee attack with the opposite end of the weapon. The weapon’s damage die for this attack is a d4, and the attack deals bludgeoning damage.
• While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter your reach.
It seems to me it should just be a typo, but I checked the Wizards of the Coast Errata and couldn't find any mention of the pike being added erroneously or left out. Is there a designer's reason for pikes not getting a bonus action to attack? All of these weapons save the quarterstaff seem unreasonable to wield the butt ends, at least to me.