How does the Boot Camp Assistant force the Windows installer to prompt the user to choose an edition of Windows to install?
The latest Windows 7 with SP1 ISO can be downloaded from the Microsoft website Download Windows 7 Disc Images (ISO Files). After entering the product key for Microsoft Windows 7 Professional Upgrade 32/64-bit (English), I was eventually lead to the page shown below. I downloaded the 64 bit Windows 7 ISO.



After mounting the ISO, I was able to determine that a  ei.cfg file does exist in the sources directory. The contents is shown below. The default is to install Windows 7 Professional.

[EditionID]
Professional
[Channel]
Retail
[VL]
0


According to Install Windows 7 on your Mac using Boot Camp, the Boot Camp Support Software includes an answer file named AutoUnattend.xml. The Microsoft document Windows Setup Edition Configuration and Product ID Files (EI.cfg and PID.txt) states the following.


  Note&nbsp;&nbsp;&nbsp;An answer file takes precedence over these files. If you use an answer file during installation, Windows Setup ignores the EI.cfg and PID.txt files.


Using the Dism command, provided by the latest Windows 10, produces the following list of images stored in the sources\intall.wim file on the Windows 7 ISO. Windows 7 Professional is the image with index of 3.

Microsoft Windows [Version 10.0.18362.418]
(c) 2019 Microsoft Corporation. All rights reserved.

X:\Sources&gt;diskpart

Microsoft DiskPart version 10.0.18362.1

Copyright (C) Microsoft Corporation.
On computer: MINWINPC

DISKPART&gt; list vol

  Volume ###  Ltr  Label        Fs     Type        Size     Status     Info
  ----------  ---  -----------  -----  ----------  -------  ---------  --------
  Volume 0     D   CCCOMA_X64F  UDF    CD-ROM      5170 MB  Healthy
  Volume 1     E   GSP1RMCPRXF  UDF    CD-ROM      2047 MB  Healthy

DISKPART&gt; exit

Leaving DiskPart...

X:\Sources&gt;Dism /Get-ImageInfo /imagefile:E:\sources\install.wim

Deployment Image Servicing and Management tool
Version: 10.0.18362.1

Details for image : E:\sources\install.wim

Index : 1
Name : Windows 7 Home Basic
Description : Windows 7 Home Basic
Size : 11,710,161,360 bytes

Index : 2
Name : Windows 7 Home Premium
Description : Windows 7 Home Premium
Size : 12,222,587,449 bytes

Index : 3
Name : Windows 7 Professional
Description : Windows 7 Professional
Size : 12,122,886,417 bytes

Index : 4
Name : Windows 7 Ultimate
Description : Windows 7 Ultimate
Size : 12,285,492,779 bytes

The operation completed successfully.

X:\Sources&gt;