why is snmpEnterprise trap binding added to standard traps

Does anyone know why net-snmp appends the enterprise specidic oid to
standard traps:
>From code agent_trap.c:
        /*
         * 'trap_vb' should point to the snmpTrapOID.0 varbind,
         *   identifying the requested trap.  If not then bomb out.
         * If it's a 'standard' trap, then we need to append an
         *   snmpEnterprise varbind (if there isn't already one).
         */

Which results in an extra binding on a trap such as linkDown
linkDown
 
 Bindings (6)
    Binding #1: sysUpTimeInstance *** (timeticks) 0 days
00h:00m:40s.97th
    Binding #2: snmpTrapOID.0 *** (OBJECT IDENTIFIER) linkDown
    Binding #3: ifIndex.26 *** (InterfaceIndex) 26 [26]
    Binding #4: ifAdminStatus.26 *** (INTEGER) up(1)
    Binding #5: ifOperStatus.26 *** (INTEGER) down(2)
    Binding #6: snmpTrapEnterprise.0 *** (OBJECT IDENTIFIER)
enterprises.8072.3.2.10

Thanks,
Joan

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