This is an extremely common, extremely well-acknowledged practice. I'm surprised the OP only just found one, because they're all over the place. The thread gets too big, a mod notes this fact, it's locked, a new one of the same name with a number is created, and the new-thread boogies commence. There is no need for it to be specified in the rules because as mentioned it's not actually a rule, and it is always pretty damn obvious why it's been locked, and there is absolutely no negative result with the possible exception of the inability to quote an old post. Get over it already.