This is a legal question, not technical. I give you the details I can but if you need further information you should bear in mind that I'm not a lawyer so my help is very limited, and I can be wrong on different points...
Also bear in mind that legal discretion is different country to country. So maybe, even if you agree with any Microsoft EULA, you may won't do any illegal -thanks to your country's law- by e.g. reverse engineering the OS, or, you may get 300 years in San Quentin for installing Win7 from the same DVD on two different computers...
Anyways. Here's your answer:
Microsoft Windows 7 PROFESSIONAL and Microsoft Windows 7 ULTIMATE both have exactly the same EULA. Both say the following:
Activation associates the use of the software with a specific computer. During activation, the software will send information about the software and the computer to Microsoft. This information includes the version, language and product key of the software(...)
Which means activation is mandatory and language information will be sent over to Microsoft to determine if you legally use the software.
The license agreement which explicitly declares you are not allowed to use MUI (but permitted to use LIP!) on Windows 7 Professional, but allowed to use on Ultimate only is described in Windows 7 and Optimized Desktop - Volume Licensing Guide (also available on MS portal). It says the following:
When you acquire Windows 7 Professional licenses, either through Volume Licensing upgrades or through an OEM, you can cover those licenses with Software Assurance to get rights to Windows 7 Enterprise, that offers unique benefits such as, BitLocker Drive Encryption and Multilingual User Interface Language Packs, as well as access to the Microsoft Desktop Optimization Pack(...)
And there's a massive table that describes what benefits you'll get if you buy Enterprise or Ultimate versions of Windows.
In other words, MUI is not "prohibited to use" in Windows 7 Professional, but instead a benefit of Ultimate and Enterprise. So Professional doesn't have this superb benefit, while Ultimate does. So if you use MUI in Professional you technically jail break the OS (or hack, or reverse engineer, etc) and by that, you break the EULA and by that the law (and can be deported to Alpha Centauri - or sentenced to jail for 500 years, probably in San Quentin, you evil madman! - just kidding! :D).
Does that make sense?