I echo this, but dude Shounen fans aren't all prepubescent brats who like guys with elongated glowing hair and shoot beams of energy from their hands either. Maybe I'm reading into your tone a little much, but that's what it seems to indicate.
Though again Kaiten is right. I've seen several Seinen that would make awesome 'Shounen', while there are some Shounen that with a little bit of a push could be 'Seinen'. Keep in mind though this is my opinion, and not based on any sort of fact. I'm not sure how the grand scheme of things works, whether a mangaka walks into Shogakukan/Shuiesha/Kodansha, and says 'Yeah I'd sorta like to draw this', and they're like 'Oh cool, well we'll sorta put it in this mag'. Or if there's a deeper clause or dichotomy that dictates what is labeled what, especially since the line between 'adult' and 'non adult' is so perfectly blurred in most media that it's hard to tell what one is getting into until they get into it, but I believe at least it just comes down to what allows the author to freely tell his or her story, and more importantly what audience is going to appreciate said story. There's a science to this, and it's more than a precedent set by people not involved in the industry can understand.