I hope this is the right forum.

I recently flew ARN - AMS.

I had one 2-way ticket with KLM (ARN-AMS, AMS-ARN)
Then I changed my plans and got a 1-way ticket with SAS (ARN-AMS)

When I tried using only my last part of my 2-way ticket KLM told me that since I hadn't used the first part the ticket was void. So I had to buy a new ticket to go AMS-ARN. In the end I got the same chair that I would have had if my ticket was not voided (same flight, same class).

Is it standard practice for airlines to void the return ticket if you don't use the first half of the ticket? I know of at least one instance where it worked fine (not KLM though).