I would have a problem if a terminal app were to do something like this, but for GUI apps, it is expected for them to make stuff easier.
And I feel like, if you were to use a slash in a file name, it would most probably be either an “or” slash or a fraction slash, so the substitution is fine in my books.
illegal characters
Not sure about calling it that, considering it is a standard UTF-8 character. (0x2044 in UTF-16)