You need to tell JMP the element from the list is a column.
dtcols = {"Score1", "Score2"};
For( i = 1, i <= N Items( dtcols ), i++,
For Each Row( As Column( dtcols[i] ) = As Column( dtcols[i] ) * 100 )
);
BTW, a simpler form to use is with a For Each() function
For Each( {col}, dtcols,
For Each Row( As Column( col ) = As Column( col ) * 100 )
);
Jim