Why can they be opened with any key?
It is up to you but a lot of us (me included) take a ST Key cut it off and mount it in knob and leave them in the panniers all the time. I have never had an issue with theft and I have been doing that for years. I do park at night in a secure area.
Looks like the photos are dead but here is a link.
I saw this farkle at OH-Stoc on someones bike and thought it was a good idea. I went to Radio Shack for knobs but wasn't really happy with the knobs or the amount of work needed to turn them into key knobs so I went looking for another alternative. I found the bottle caps from certain Yamalube...
www.st-owners.com
This link is for info on disassembling the panniers. I was thinking there might be a better image of the lock to see if those might fit. Maybe it will help though.
I really don't know if this has been done on here. Tried looking for something similar but found nothing. So I'm going to do an entire writeup including pictures of taking apart the bag locking mech along with the key mech and the paint work, before and after, etc etc. I've really been wanting...
www.st-owners.com