Yes,You are right.If we write statement as :
Select-options : vbeln for vbrk-vbeln obligatory.
then low value gets mandatory.

We have to write as :
Select-options : vbeln for vbrk-vbeln.
Now vbeln field is not mandatory.
Now we have to explicitly write code for making high value 
mandatory in AT SELECTION-SCREEN event.

At selection-screen.
loop at vbeln where high is initial.
endloop.
IF SY-SUBRC ne 0.
 MESSAGE 'HIGH VALUE CAN NOT BE BLANK' TYPE 'E'.
ENDIF.

There is only one change required in the given code and 
that is IF SY-SUBRC eq 0.
Then the code will look like this

At selection-screen.
loop at vbeln where high is initial.
endloop.
IF SY-SUBRC EQ 0.
 MESSAGE 'HIGH VALUE CAN NOT BE BLANK' TYPE 'E'.
ENDIF.

SELECT-OPTIONS s_matnr for mara-matnr.

AT SELECTION-SCREEN.
  if s_matnr-high is INITIAL.
    message  'high cant be empty' TYPE 'E'.
  ENDIF.