On the player facing-side, I can see why this is troubling, but I'm convinced it's not a bug because the game is actually doing what it's supposed to do correctly.

You've got two basic ways of programming the effect of Scallywag into the game. (A) Have Scallywag, even upon death, exist in some form, granting an aura that tells the game that any minions summoned to the battlefield from the deathrattle event should attack immediately. (B) Simply hard-code the minion to attack immediately whenever it enters the battlefield.

In the Hearthstone engine, (A) is extremely problematic to actually code in. Once they die, minions can no longer produce auras that affect the board and minions on it. They're moved to some graveyard array listing minions that died that game but in the gamestate's evaluation process, they are no longer in existence. With Scallywag's summon a deathrattle effect, the way that Hearthstone processes events is that deathrattles are resolved and the minion moved to this graveyard array before any attacks can occur. Thus, if it simply granted an aura telling the token to attack immediately, this aura would never actually take effect, as Scallywag is in the graveyard; the game is no longer processing the auras that it would produce; and so the token wouldn't attack. Implementing this would require writing a bunch of new code, including how the active gamestate evaluates minions in the graveyard.

(B) is a much easier option. Just have the token have the innate characteristic to attack immediately, in the same way that we've seen cards summoned on deathrattle have rush/taunt/whatever. In this case, because it's Battlegrounds and they never expected it to be returnable to the player's hand or even to really exist for more than a second or two, they chose to simply not have the token display on it's card that it can attack immediately, as that would require extra work.

So, when the token gets played, the game looks at it and reads a bit of code that says that it should attack something immediately. Because the implementation of the Tavern is borrowed code from the normal gameboard, it does so. It's working perfectly as intended, namely that when it enters the battlefield, it does so. What the devs never foresaw was it finding a way to enter the battlefield in any instance other than being summoned as a deathrattle effect from Scallywag.