Installing OSX 10.7 and later in VMWare Fusion on OSX is trivial. If I recall correctly, VMWare does it automatically if you just pass it the Installer from the app store.

Installing 10.6 requires a bit of hackery unless you have a copy of the server version.

I have a bunch of VMs for testing my OS X apps on older versions of the OS.

Apple only permits you to virtualize their OS on Mac hardware. They make it purposely difficult on everything else as they want every developer to have to buy into the hardware ecosystem to be able to build software for OS X and iOS.

Apple allows people to virtualise OS X as a guest OS on top of an OS X host OS, on Mac Hardware.

Both commercial virtualisation platforms of OS X (VMWare Fusion and Parallels Desktop) have very easy "Create OS X VM from Recovery Partition" functionality.

Apple didn't make it a pain in the ass - if you need patched ISOs to install, it sounds like you're trying to do something they specifically don't allow.

I managed to install OS X Yosemite on Windows VMWare Workstation 10 using a commonly available VMWare patch and a USB stick for the installation, created using the standard createinstallmedia command.