Here's why:
* Royal Decree negates the effects of Spell and Trap cards.
* Counter Trap cards are a sub-type of Trap cards.
* Counter Traps activate in response to a Spell or Trap being activated, not when they are on the field.
Therefore, Royal Decree will not prevent Counter Trap cards from activating, even though they are Trap cards.
Example:
1. You activate a Spell card.
2. Your opponent activates a Counter Trap card like Magic Drain in response to your Spell.
3. Royal Decree on the field will not stop Magic Drain from negating your Spell and giving your opponent LP equal to the Spell's activation cost.
However, Royal Decree could prevent Counter Trap cards from being activated if:
* The Counter Trap is chained to a Spell or Trap card that Royal Decree negates.
* The Counter Trap has a continuous effect that Royal Decree could negate.
Important Note:
Some Counter Trap cards have special effects that may not be affected by Royal Decree. For example, Solemn Warning can be activated in response to Spell, Trap, or Monster effects, meaning it cannot be negated by Royal Decree.