Why doesn't IIS 7.5 FTP respect the passive port range for plain ftp?

I stumbled upon this question when I had the same problem today. I set the "Data Channel Port Range" in IIS 7.5 Manager console, but the server kept picking ports outside the specified range. The issue for me wasn't a firewall rewriting the response (I was using TLS, so it couldn't, even if it tried). Actually, after you change the port range, you have to restart the "Microsoft FTP Service". Using iisreset isn't sufficient, nor is clicking "Restart" at the server level in IIS Manager; the OS-level service (ftpsvc) must be fully stopped and started.

I eventually determined that the $%(*&@ firewall was REWRITING the ftp commands in an attempt to be "helpful". I determined this when I tried to install FileZilla server and had the same problem. Luckily, FileZilla server has an open monitoring window, and I was able to observe the conversation from both sides and determine that the server was, in fact, sending the proper port number.