sure what the point of taking additional time and effort to trace out the source would do. I believe that the majority of SPAM on the Internet is generated from co-opted home computers, which means that the "sent by" IPs are always changing and are in the tens of thousands, if not 100's of thousands and more.
Believe me, it's enough labor to zap each and every SPAM post, let alone trying to track down IPs that are masked by false "sent by" tagging, just to think you know where they are originating from.
Jim