As far as I'm aware, there aren't any official rules about rushing. Early game rushes and all-ins don't work nearly as well against the high level players as they do against the weaker or casual players. A player who has very good control over his drones can easily defeat a 6 pool, for example.
Also, rushing is very hard on the economy, for no guaranteed reward. You say that the lings could easily destroy the second base, but that's not always true. Unless the opponent is doing a high risk early second base, such as Nexus first protoss, then they almost always will have at least some way to try and defend such an attack. If the rusher doesn't do significant damage, then they are stuck with fewer workers, wasted resources on the assault, and only on a single base, which means a significantly harder middle game.
The size of the map is also a large factor, as well as how early the player scouts and gets scouted. With larger maps, rushes become very risky as it takes longer to reach the opponent's base, giving them time to get units of their own, not to mention the time it takes to scout out said opponent's position.
There are certainly more factors involved, including the players' predisposition to rushes, but the players that reach that level of play have generally done so by mastering a combination of macro and micro. Rushes get punished much more frequently, and so most players don't believe that the risk is worth the reward at that point.