Its because if the file exists, bash uses your local settings for the readline settings instead of the default one.
From http://www.gnu.org/software/bash/manual/html_node/Readline-Init-File.html
"When a program which uses the Readline library starts up, the init file is read, and the key bindings are set."
The recommendation in the sample file at gnu.org is to source the default in your local one with a line like this:
$include /etc/inputrc
and then modify from there.