Because I just talked about it in chat: the link points to an emulator for IBM /370 and /390 hardware. And IBM supplies operating systems for it - old ones of course, but completely legal. OS/360 or MVS 3.8j or even an old VM/370 version on a Linux box is really a neat toy for the geek from back then
And as I just see there's now also an OS X version. Very nice.