Well, let's take a peek at a few things here...
Level Limit - Area B
Continuous Spell
All face-up Level 4 or higher monsters on the field are in Defense Position while this card is active.
Notice this indicates that the monsters must remain in defense position. A monster would have to be in ATK position (minus that one guy who escapes my mind) in order for them to be a valid monster to attack. Since Horus LV6 is unaffected by spells, this is why it gets through.
Gravity Bind
Continuous Trap
All Level 4 or higher monsters on the field cannot attack.
Again, this trap card prevents monsters on the field from attacking. Since Wildheart is unaffected by traps Wildheart would still be a valid monster to ATK at this stage.
Threatening Roar
Normal Trap
Your opponent cannot declare an attack during this turn.
Now this trap goes up the chain one step further and prevents the player from even declaring an attack. This doesn't affect the monster at all.
Remeber, when you enter the Battle Phase, the player must declare the attack with an eligible monster to attack. The monsters just don't attack on their own. Otherwise
Berserk Gorilla would come into question with
Swords of Revealing Light and such, but this is not the case. If all you have are defense position monsters, you have no eligible monsters to attack, then you can't even declare an attack. LLAB, GBind make monsters inelibigle for an attack.
Notice a difference with a different staller...
Messenger of Peace
Continuous Spell
Neither player can declare attacks with face-up monsters with an ATK of 1500 or more. Pay 100 Life Points during each of your Standby Phases. If you do not, destroy this card.
This is another card that is preventing the player from declaring an attack with a monster that exceeds the 1500 ATK barrier. Notice the restriction is on the player and not the monster.
Which brings me back to
Threatening Roar. Notice one ruling about this card:
You can activate "Threatening Roar" in response to an attack, but the attack has already been declared so it will resolve normally. After that, your opponent cannot attack for the rest of the turn.
Now in a battle situation with
Gravity Bind, the attack is effectively negated at this point because the monster is no longer eligible to attack. But with
Threatening Roar, there was nothing done to stop the monster from attacking, it was stopping the player from declaring any further attacks.
So that's where I'm coming from. There is a separation between the declaration of the attack (in which you need to have an eligible monster) and the monster actually attacking (where GBind and others revoke that eligibility)
Interestingly enough, I'm tempted to visit LLAB with Horus LV6 again. The emphasis is on the player, but I'm tempted to think that since the card also involves a monster in its text, LV 6 gets to sneak by.