Aim assist for snipers works different than aim assist for hand cannons. Snipers sights stick to opponents. (Think of it in milliseconds since thats usually how long it will stick). It DOES NOT grant a bigger hitbox on your targets so you still got to be a good shot to land hits and crits. Aim assist however does increase hitbox so slightly for all other weapons. If you do not believe or are stuck on your logic that snipers take no skill. Please visit planet destiny. They have a full guide on how AA works. :)