The version of /etc/bashrc contained in bash-1.14.7-13 is as
follows:

  # /etc/bashrc

  # System wide functions and aliases
  # Environment stuff goes in /etc/profile

  # For some unknown reason bash refuses to inherit
  # PS1 in some circumstances that I can't figure out.
  # Putting PS1 here ensures that it gets loaded every time.
  PS1="[\u@\h \W]\\$ "

  alias which="type -path"

RTFM.

Searching the BASH(1) man page for the string "PS1" gets a
hit in the INVOCATION section, which reads in part:

"PS1 is set ... if bash is interactive, allowing a shell
script or a startup file to test this state."

Setting PS1 unconditionally, as is done by the current
/etc/bashrc, breaks this facility.

If it is desired to set PS1, when appropriate, to a specific
string, this can be done with:

  # PS1 is only supposed to be set for an interactive shell.
  # Since this is the only easy test for an
interactive        # shell, it should not be tampered with.
  #
  if [ x${PS1+set} = xset ]; then
      # interactive
      PS1="[\u@\h \W]\\$ "
  fi

Three reasons why a user should be able to use a simple test
(namely, seeing whether PS1 is set) for whether the current
shell is interactive:

3. ``stty </dev/tty erase ^H'' gets an annoying complaint
if    the shell is not interactive.

2. Operations such as updating the window title bar
generate    unacceptable overhead if done in a long-running
script.

1. BASH(1) SAYS YOU CAN DO IT.