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 parallel plans 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 parallel plan?

Is this an oversight, a bug or intended?

Regards
Thomas