Krusty I think you'll find it has more to do with the manufacture "locking" certain pipelines. I've seen several chips that either shipped "unlocked" or could be unlocked by the average user.
As to why, well I'm no rocket scientists like some of these guys are.
I suspect its a bit like throttling down an engine. Run it wide open and it may overheat, may not perform as well, or it may go poof before the warranty is up.