That's an excellent question. It doesn't make sense other than a bug of some sort. Here's a work around for PowerShell:

cmd /c 'ftp -s:c:\ftp.txt'

I stumbled upon a fix:

ftp '-s:ftp.txt'

Not sure why this is required for this parameter other than maybe something with the PowerShell parser that doesn't like the punctuation.

the PowerShell parser sees the -s and tries to parse it as a PowerShell parameter when it really isnt'. Use my fix to get around this. Another alternative is to use the free FTP com object from SAPIEN (SAPIEN Technologies, Inc. - VBScript Editor, PowerShell Editor, ASP Editor, ActionScript Editor, PHP Editor, and more.). Shay has some nice PowerShell functions also ready to go for this at $cript Fanatic: FTP transfers automation.