It is given as a knock-on as the player has dropped the ball and failed to bring the ball back into play in the prescribed manner. If this was a deliberate act then it would be a penalty for that offence but as it is accidental then the same as for a forward pass the scrum is given.

Who are you addressing here?

Calm down, no-one in particular, just the average fan who holds an opinionated voice about the rules without ever reading them, and/or their interpretation without ever trying to apply them.

I am calm, don't worry. I've taken the Match Officials course so I'm alright!

But that's not what the law says, it makes no distinction between deliberate and accidental and a referee cannot determine intent.

Because a deliberate drop would have to be played immediately (backwards) by the foot. A player accidentally dropping the ball will not have time or presence of mind to do this so it invariably won't be a correct PTB anyway.