If anything,I think IPB's method is wrong. You get a falsley inflated post count. If the posts are no longer there, why should they still be counted? I'll admit, I hate seeing my post count here drop due to removal of threads as much as any other webmaster, but I'd also much rather have a true count of the posts on the forum.
Also, IPB forum admins can always go into their admin panel and have the board do a recount, which would result in the correct post count.