At the risk of generalizing, it seems to me that Japanese people are far more skittish about uploading scans onto the Internet than foreigners. I'm not sure that it's ever happened with magazines, but there have certainly been high profile cases of people being imprisoned for pirating manga, so I can understand the reluctance. Of course, pirating manga is a direct financial blow to the publishers/creators, since the manga is usually still being sold in some fashion. Whereas the only places selling old magazines are used bookstores and auction sites, so it isn't harming the publishers in my opinion. Still, I wouldn't be surprised if the attention being brought to your scans was what scared that guy off of twitter.