Check-in luggage quota

Many airlines extend the luggage quota on tran-atlantic flights, I never understood why, and correct me if I'm wrong- the flights you are talking about are transatlantic.

Within the US you are allowed 2 suitcase. Outside the US you are allowed 1 suitcase. For flights going to or coming from the US, there are agreements in place allowing travellers the US limit. 

Why do non-US airlines only allow 1 suitcase? My guess is to make money by charging for overweight luggage. 

Flights to and from the US use the "piece system," which typically allows two checked bags of 23 kg or 32 kg.  Many other flights, such as those between Europe and Asia, use the "weight system," which typically allows one checked bag of 20 or 30 kg.

Here are two good links on the subject:

OAG: Baggage Allowances

IHT: Frequent Traveler: The baggage blues and how to cope

Most international airlines belong to IATA which is an International Air Transport Association. These airlines agree to abide by IATA regulations. Because fuel costs have gone up astronomically IATA members have instituted strict regulations to reduce weight and thus save on fuel costs.