Because you were flying to/from Argentina, Brasil, Canada or the US. For an overweight piece of baggage (between 23 and 32 kg) they "only" ask for 65$ (50$ if booked online). An extra checked bag would've been 250$ (200$).
On all other routes, you have to pay every additional kilo. On LHR-DXB, you would have to pay 10x 30$ =300$ (or 250$ online).