Are you sure about that? I have gotten many spoofed spam emails before but I've never seen them appear in my sent folder before. Furthermore when I search "from:[my email]", I see a different listing than when I look in the sent folder.
I have a very short gmail address so I get _loads_ of spam daily. The above is actually correct. Emails from my address, even though they are spoofed will show up in my Sent folder until I mark them as Junk. I just checked with the 3-4 spams I received last night all all where in Sent.
It's non-standard, for sure, but it's "by design" and I suppose they can do whatever they want on their own platform. I'll stick with my IMAP4-compliant mail server, though.