As churnd notes, it's a default entity, though it's not on /Users, but on the home directories themselves, populated under /Users, intended to prevent accidental deletion.
It's there by default during creation of home dirs, because new home directories are created based on a template found in /System/Library/User Template. It's these files that have their ACLs specified in a property list.
On 10.6, in /var/db/receipts directory, the default ACLs are specified in property lists. Below is an example, where I'm using the defaults command to read the PathACLs key in one of the primary .plist files created when the OS was initially installed.
[root@lithium5 19:13:43 /var/db/receipts]# defaults read /var/db/receipts/com.apple.pkg.Essentials PathACLs
{
Applications = "!#acl 1\\ngroup:ABCDEFAB-CDEF-ABCD-EFAB-CDEF0000000C:everyone:12:deny:delete\\n";
"Applications/Utilities" = "!#acl 1\\ngroup:ABCDEFAB-CDEF-ABCD-EFAB-CDEF0000000C:everyone:12:deny:delete\\n";
Library = "!#acl 1\\ngroup:ABCDEFAB-CDEF-ABCD-EFAB-CDEF0000000C:everyone:12:deny:delete\\n";
"System/Library/User Template/English.lproj/Desktop" = "!#acl 1\\ngroup:ABCDEFAB-CDEF-ABCD-EFAB-CDEF0000000C:everyone:12:deny:delete\\n";
"System/Library/User Template/English.lproj/Documents" = "!#acl 1\\ngroup:ABCDEFAB-CDEF-ABCD-EFAB-CDEF0000000C:everyone:12:deny:delete\\n";
"System/Library/User Template/English.lproj/Downloads" = "!#acl 1\\ngroup:ABCDEFAB-CDEF-ABCD-EFAB-CDEF0000000C:everyone:12:deny:delete\\n";
"System/Library/User Template/English.lproj/Library" = "!#acl 1\\ngroup:ABCDEFAB-CDEF-ABCD-EFAB-CDEF0000000C:everyone:12:deny:delete\\n";
This example snippit shows group:deny everyone delete for things like /Applications directory, but also folders in /System/Library/User Template/English — it's that folder that is the source for how new home directories appear when created using the PrefPane or createhomedir command.