This method should be called updateTags. Can you explain that if this method is called without first retrieving the tags it will delete the existing tags as per the Connections API documentation i.e.

To update the tags that one user created and assigned to another user's profile, use a HTTP PUT request to update the tags listed in the tag creator's tags category document.

When you update profile tags, the existing tag information is replaced with the new data. To avoid deleting all existing tags, retrieve the tags that you want to retain first, and send them back with this request. See Retrieving profile tags for more information.