I suggest you launch a bug for it in the bugtracker then as that is the only way things will/might be resolved permanently.
This is NOTa bug, please don't report anything, it's the way turba works and has worked for quite a long time. You can check the turba archives and see that I asked the same question a couple of releases ago as to why they changed the behavior and what the horde devs response was.
If anything, you might could ask on the horde list as to whether these messages can be suppressed. I honestly think someone has and the answer at that time was no.
John