How preserve JPEG date stamps during Automator scaling?
Here is one way of achieving the stated goal with an Automator Service. Instead of using Scale Images use Run Shell Script with settings, Shell: /bin/bash and Pass input: As arguments and add the following code to it:

for f in "$@"; do

        # Get the creation date time stamp of the target file, saved as 't'.

    t="$(/usr/bin/GetFileInfo -d "$f")"

        # Get the pixel width of the target file and divide it by 2 for a 50% scaling, saved as 'w'.

    w="$(/usr/bin/sips -g pixelWidth "$f" | /usr/bin/grep pixelWidth | /usr/bin/awk '{print $2/2}')"

        # Scale the target file.

    /usr/bin/sips --resampleWidth $w "$f"

        # Set the modified and creation date time stamps of the target file to the saved value held in 't'.

    /usr/bin/SetFile -m "$t" -d "$t" "$f"

done
        # Notify User operation is finished.
/usr/bin/afplay "/System/Library/Sounds/Purr.aiff"


Note that the last line in the code above can be omitted if you are going to use Show Growl Notification, which I don't have and why I used a system sound.  This was done under OS X 10.8.5 and you'll need to verify the path to the executables are the same on your system, although they should be the same.  The reason I'm using the fully qualified pathname of each executable is because Automator errors out if I don't.  You can get the FQP in a Terminal using which, e.g., which sips returns /usr/bin/sips.

Note that GetFileInfo and SetFile are a part of Xcode Tools and if not already installed you'll need to install, which can be triggered by typing GetFileInfo in a Terminal and following through with the prompts.

You can read the manual pages for each of the commands used to garner a more through understanding of what's happening.

Manual Pages for: GetFileInfo, sips and SetFile