From The Guardian :
www.theguardian.com
"Caroline Normand, the consumer group’s director of advocacy, said: “It’s totally unreasonable for an airline to cancel a passenger’s return flight – often without warning – simply because they’ve missed the first leg of their journey. "
"The airlines that retained the clauses were British Airways, Virgin Atlantic, Emirates, KLM, Air France, Singapore, Qatar and Swiss."
Another thing to be aware of when buying tickets with a return flight.

Airlines refuse to scrap ‘no-show’ clauses for passengers
BA is among firms that bump passengers off second leg of return trip, despite criticism
"Caroline Normand, the consumer group’s director of advocacy, said: “It’s totally unreasonable for an airline to cancel a passenger’s return flight – often without warning – simply because they’ve missed the first leg of their journey. "
"The airlines that retained the clauses were British Airways, Virgin Atlantic, Emirates, KLM, Air France, Singapore, Qatar and Swiss."
Another thing to be aware of when buying tickets with a return flight.