The same thing goes for VirtualBox but they (Oracle) do it differently. VMware and Parallels enforce it by taking technical measures so that their software will only recognize and allow OS X Server dvd's. Oracle doesn't, they simply warn the user about the EULA violation if they install anything else than OS X Server.Neither VMWare nor Parallels want to get on the bad side of Apple, that's why they only support virtualization for Mac OS X Server (where the EULA explicitly allows it).
It's about the EULA and how you as a company want to deal with it. You could descibe this as "politics". I think VMware and Parallels want to play it a lot safer than Oracle. I'd vote for the legal department vetoing it.It's just politics and maybe their legal department vetoing.