This is likely because the users mentioned have been added to a group named Work Item Only View Users found at the server level on TFS (in TFS 2010). Here is a blog post that covers this well.
Adding users to this group sets it up so that they can only see the ones they create. It also means that they do not require a Client Access License (CAL) for TFS; You can remove them from this group, which will imply that your company already has the CALs that cover these users. For information about the licensing, please see the latest version of the Visual Studio 2010 Licensing Whitepaper, last updated April 2012. See the section titled When A Client License is not Required.
If you are not worried about the CALs so much because your users are covered, etc... then you can take them out of that group and limit their permissions with a fair level of granularity. I can't post more links on this site yet because this is my first post on superuser, but you can check out the topic Team Foundation Server Permissions on Microsoft's MSDN site for details about what is possible.