If you look at this article, there are ways to install any version from the media. It just requires changing one file and thus copying the media. So, the immediate answer is that you can't do what you want but with a little work you can.
I suspect the reason for this change comes from the way product keys are handled. With Vista you had to enter the product key before installing Windows. With Win7, you don't have to enter the product key up front. If users were allowed to choose what to install, they might choose ultimate and then find out 30 days later that their key is only for Home Premium.
(edit - see accepted answer)This should be the solution but it's not, it did work once but no thereafter. To answer my question, YES (well, once!) it is possible to map/mount a network drive (presumably by Samba/SMB) as a drive letter in Windows 7 HOME (and likely same procedure for Professional, also, I will update).
It should be a simple standard procedure, once you know, and here's how.
Pre-requisite: for your network drive/NAS drive brand, you may require a utiliy to be installed first. With Lacie drives, such a utility is the Lacie Ethernet Agent/Lacie Ethernet Assistant. This application should be running in your system tray all the time, once installed.
1) Start Menu, Select the Computer option to open the Computer Explorer Window. On the second-level drop-down menu options, find the 'Map Network Drive' and select it:
2) Select an available/free drive letter (in my case Z:) and enter the server and path, in my case this is \Lacie-2big\lacie-2big-raid
3) DONE! An explorer window should open showing the contents of your drive.
Best Answer
Follow the How do I install gpedit.msc on Windows Starter Edition, Home and Home Premium? instructions to get gpedit.msc installed onto Windows 7 Home Premium, and once you have that complete, you can then access the Security Settings via gpedit.msc to apply the same settings there as the Local Security Policy allows via secpol.msc.