The constraint is there to prevent people from making trading calls and then going back later on and editing the direction of their call if things go against them. It's also there to prevent someone from starting a popular thread with a lot of replies and then deciding days later to delete the whole thread.
That is an older mindset. CJtheTrader there is talking about deleting their OP an hour after they posted it.
WTF Baron!?!
Someone creates a thread they regret? They should be able to delete it within a day or two. Not all regretful threads are trading threads where they are "trying to cheat and seem they are better traders than the OP."
WTF.