Airline Price Fixing Fines: Prosecutors Target 21 Companies Over Passenger, Cargo Fees

Still don't get from the story where price-fixi­ng is coming from. Coming together to "fix" is treated conspirato­rially in this article, yet that is done all the time by businesses to protect common interests. What does the law say about pricing for instance? That's not spelled out here in my opinion. While never on the side of corporates­, what I'm reading about the US government punishing is partnershi­p (albeit of a strategic kind). Help with the gaps please. Thanks.
Read the Article at HuffingtonPost