That is not enitrely correct. You are only allowed to virtualise 10.7 or 10.8 when you have bought it via the Mac App Store. When it came with the computer you are out of luck; if you want to virtualise you need to buy it from the MAS.
Also, there is nothing in the license for 10.7 nor 10.8 that explicitly tells you that you may only run it on the same version. It only says that you are only allowed to run "the Apple Software" on "the Apple Software" and thus it comes down to how one interprets things. Just to show you, here are some of the relevant pieces from the SLA:
From the 10.7 SLA:
(iii) to install, use and run up to two (2) additional copies or instances of the Apple Software within virtual operating system environments on each Mac Computer you own or control that is already running the Apple Software.
"the Apple Software" is defined as such (the SLA starts with it):
The Apple software (including Boot ROM code), any third party software, documentation, interfaces, content, fonts and any data accompanying this License whether preinstalled on Apple-branded hardware, on disk, in read only memory, on any other media or in any other form (collectively the “Apple Software”) are licensed, not sold, to you by Apple Inc. (“Apple”) for use only under the terms of this License.
From the 10.8 SLA:
(iii) to install, use and run up to two (2) additional copies or instances of the Apple Software within virtual operating system environments on each Mac Computer you own or control that is already running the Apple Software, for purposes of: (a) software development; (b) testing during software development; (c) using OS X Server; or (d) personal, non-commercial use.
"the Apple Software" is defined as such (the SLA starts with it):
The Apple software (including Boot ROM code), any third party software, documentation, interfaces, content, fonts and any data accompanying this License whether preinstalled on Apple-branded hardware, on disk, in read only memory, on any other media or in any other form (collectively the “Apple Software”) are licensed, not sold, to you by Apple Inc. (“Apple”) for use only under the terms of this License.
The problem is interpreting what is defined as "the Apple Software". Is it OS X, is it this specific version of it ? There is no proper definition of what it is other than what's in the above quotes. The definition is rather "wide", you could interpret it as being any OS X version. I think that's the entire idea behind it: you are only allowed to run the OS X client version on an OS X client. The OS X server version is different since you can run it on both client and server versions of OS X as well as ESXi. The licensing is not as simple as you are depicting it here. This is where some help from a lawyer would be very helpful. Point one at http://apple.com/legal/sla and ask for advice.
A more practical take: just run the latest OS X version you have. 10.8 is better than 10.7 on various levels anyway 