On topic, if I was ever running a company that sells fish, the fish I would give away under those "customer appreciation" circumstances, even if i was not even thinking about the fishes well being, it wouldn't be an oscar.
Let's face it, even though oscars are common as can be, they can still be sold for good money. And that's the biggest part of running a business. Making money.
If there were a fish to give as "costumer appreciation" once again, well being of fish not involved, it'd likely be a common livebearer of sorts or a small danio. You know. Cheap things that you can throw in there for free and say "thank you for giving us money, now have a cheap fish that we'd only make an extra dime on" not "We're giving away oscar fish so they can eat the fish you just bought and grow too large for your tank"