Command Line FTP Client Confused By Tilde?
What you are seeing is expected behavior.
The tilde (~) is a part of the shell's standard syntax that is synonymous with the user's home directory.

Tilde expansion is the process of converting these abbreviations to
the directory names that they stand for.

When you are in FTP, you are not using your shell (bash, csh, zsh, etc.), rather you are using the FTP client and it has no idea what the tilde means and thusly, it cannot expand it out.
~ and HOME are the same thing.  If you execute the command
$ echo ~ $HOME

You should get your home directory output twice.  (The $ is for variable expansion)
/Users/fweth /Users/fweth


Trivia
The convention of using the tilde (~) derives from the Lear Siegler ADM-3A terminal that had a keyboard shortcut for the the users' HOME on the same key as the tilde.