In nature a pair of adult jack dempseys will hold a territory the size of an SUV, somewhere between 500 and 1000 gallons, and the pair will chase any other cichlid away. In the pic below the rock is about the size of a VW Beetle, the depth from surface to bottom, is about 10-15ft, and an interloper is being chased out by the male, other non cichlids are usually ignored (those live bearers hovering near the surface), although the catfish in a lower left which preys on JD fry, is watched like a hawk.

Does this mean a pair of JDs should be given a 500 gal tank? Many consider a 55 adequate for a pair, because aquariums, by nature, are unnatural environments.
Is a 75 gal too small for Oscar? An adult grows larger than a JD?
Would you like to spend a lifetime in a bathroom, or a room the size of a jail cell?
Can the Oscar survive adequately in a 75? Of course, but is it cruel and unusual punishment?
What do you think an Oscar territory in S America would be, a 75 gal puddle, or a few thousand gallons or potentially millions, in a river?
Is it ethical to hold something large just because we like it, in what "it" might consider a puddle, or would it be better to choose something smaller that would be an ethically more realistic choice.
All questions we aquarists should probably be asking asking ourselves every day.