there's nothing wrong with an original poster thanking someone who helped in a thread
but consider this scenario:
A asks a question, B replies (this could take several posts each, with perhaps other helpful posts added by other parties C, D, and E), and then, about six months later, Z comes along, who previously had no involvement in the topic, and says "thanks for this"
this is egregious forum behaviour, because now the thread comes up again on the daily list, and people will be curious to see what new information has been added to an interesting thread, but when they open the thread, there's nothing really new, is there
it wastes everybody's time