I found two articles (in Austrian) (in German, from a newspaper that is nothing serious):
As of 2010 and 2013, respectively. If I understand them correctly, some Austrian court ruled that Austrian Airlines and Lufthansa can not prevent passengers, who missed the first flight, from boarding the return flight. (The first article says that in this case, passengers must pay the price difference to a one-way ticket, the second article says that passengers no longer have to pay the difference in price).
What is the situation today? Are there similar opinions in other countries (possibly from the EU)?
So, my question is: Which airlines allow (and in what circumstances) to use only the return flight?