The airline is correct. They can (and will) cancel all remaining segments if you miss your outbound (or any subsequent) segments.

I suppose you could try to sue (though I don't think you'd win), but that doesn't really resolve your problem. You'd still be at your destination without a ticket to get to your origin.

Your best bet is to see if you can either change your original ticket, or refund it and use the money to buy a ticket that fits with your new travel plans. There will likely be fees to do either of these, but it will be cheaper than trying to resolve it at the airport when the airline tells you that youhave to buy a new ticket to get home.