So, i was reported same issue in 7.0 b4 if i remember correctly
and it was fixed in 7.0 b5
but, looks like programmers working at Mikortik forget what was done and when
so in next 7.1 b it was lost again
here is the original post
http://forum.mikrotik.com/t/ros-7beta-and-usb-ethernet/135299/3