Execution plan using B+ tree index, but also sorts
Oracle says about Indexes and Index-Organized Tables under Full Index Scan: In a full index scan, the database reads the entire index in order. 

Yet, unter Fast Full Index Scan, it reads: A fast full index scan is a full index scan in which the database accesses the data in the index itself without accessing the table, and the database reads the index blocks in no particular order. (Emphasis mine)

Now, probably, the question should be: why did the optimzier choose INDEX FAST FULL SCAN over INDEX FULL SCAN.

A hint to the answer of the latter question is given in  under 11.2.3.7 Fast Full Index Scans:  A fast full scan is faster than a normal full index scan because it can use multiblock I/O and can run in parallel just like a table scan.

If you insist that Oracle use a full index scan, you might want to try the /*+ index() */ hint:

create table tq84_foo (
  fooID number not null
);

create table tq84_bar (
  fooID number not null
);

create unique index ix_foo on tq84_foo(fooID);
create unique index ix_bar on tq84_bar(fooID);

explain plan for
 select /*+ index(f ix_foo) */fooID from tq84_foo f
        MINUS
select /*+ index(b ix_bar) */ fooID from tq84_bar b;

select * from table(dbms_xplan.display);


resulting in

------------------------------------------------------------------------------
| Id  | Operation           | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |        |     1 |    26 |     4  (75)| 00:00:01 |
|   1 |  MINUS              |        |       |       |            |          |
|   2 |   SORT UNIQUE NOSORT|        |     1 |    13 |     2  (50)| 00:00:01 |
|   3 |    INDEX FULL SCAN  | IX_FOO |     1 |    13 |     1   (0)| 00:00:01 |
|   4 |   SORT UNIQUE NOSORT|        |     1 |    13 |     2  (50)| 00:00:01 |
|   5 |    INDEX FULL SCAN  | IX_BAR |     1 |    13 |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------