It's quite simple:  readProcess uses the low level exec*() series of functions under the hood.  Quoting is assumed to have already been dealt with; every parameter is a separate String (or (char *) in the C API) and passed literally.

Quoting is used at the shell level (and shell-based APIs such as system()) to enable the shell to correctly generate a list of literal (char *) parameters.

In general, if an exec-style API takes a list of strings (e.g. C execve(), Haskell readProcess, Perl's multiple-parameter form of system), it's using the exec()-based API and you should pass argument strings exactly as you want the program to see them; if its a single string, it's using a system()-based API and you need to worry about quoting.  In this case, the tip-off is that the argument list is a [String] and not simply a String.