So your logic is it's okay to mistreat a creature because someone else would likely mistreat it more?
It looks pretty bad when you boil it down.
not mistreat.
I'm not saying it's okay to keep a fish in a space way too small for it to grow it's full potential or be able to move around significantly. But I am saying why not get a fish if it can grow its full length and still be able to live decently and fully, with the water always being in pristine condition and the diet being proper, the tank mates being compatible, and the stock not being overbearing, even if your tank isn't quite the size it would prefer? If it really mistreating an animal if you're taking care of it correctly, even if your tank isn't quite a recommended size? Are you mistreating a cat that would like to go outdoors by keeping it inside if you live in a studio apartment, even if you feed it well and keep the place clean and allow him to roam the apartment freely?
I do understand what you're saying, and I don't fully disagree.
At the same time, I don't think it's wise to look to other people's behavior to determine your own. Yes, there are many bettas in the world that are kept in terrible conditions. But my responsibility is to the one betta that I own, which is why he has a 10 gallon tank.
Unfortunately, there are no laws that regulate the minimum environment for fish. But no one needs a betta or a zero danio. If you want to 'rescue' fish, wouldn't it be better to rescue fish that you CAN provide the proper environment for, including the proper amount of space? Let's say you have a 20 gallon tank. Instead of rescuing a danio, rescue a rasbora, for example---there are just as many of them kept in improper conditions. Otherwise you're really just excusing your choice to keep a danio in a cramped space by saying other people do worse.
I don't think it's outright cruelty to keep a fish in a smaller space than recommended. But I do think it's being slightly foggy to claim you're doing it for the fish's sake instead of just because you wanted that particular species over the many others that could be comfortable in your tank size.