If you're changing any bits within the contents of a file (whether that is via tagging or whatever), then I would expected the modified date of the file to change, so I think the current behaviour is correct. For example, if I was to do an incremental backup of my device based on timestamps, I would want it to include the newly tagged images.

If this upsets you, then maybe what you want is a way to add tags (and rotation info) only to a separate database, and not to alter the images themselves. I can see that for some people that might be desirable, but not for others. I guess it could be an option.