9.6beta, parallel execution and cpu_tuple_cost

Hello,

while playing around with the parallel aggregates and seq scan in 9.6beta I noticed that Postgres will stop using
parallelplans when cpu_tuple_cost is set to a very small number. 

When using the defaults and max_parallel_degree = 4, the following (test) query will be executed with 4 workers

   explain (analyze, verbose)
   select o.customer_id,
          count(*) num_orders,
          sum(ol.price) as total_price,
          sum(p.purchase_price) as total_purchase_price
   from orders o
     join order_line ol on o.id = ol.order_id
     join product p ON ol.product_id = p.id
   group by o.customer_id;

The execution plan is: https://explain.depesz.com/s/C7g

After setting cpu_tuple_cost to something small:

   set cpu_tuple_cost = 0.0001;

No parallel wokers are used: https://explain.depesz.com/s/q1zb

I am not sure I understand why this is happening. Why would lowering the CPU cost for a tuple result in not using a
parallelplan? 

Is this an oversight, a bug or intended?


Regards
Thomas