Because humans are barely sapient animals with limited understanding of ourselves and little to no awareness of the long-term consequences of our actions.

We don't operate in our own best interest or the best interest of the group, we're built on the assumption that the environment and our local community will moderate our actions. There's natural limits to physical actions, natural repercussions to social ones when everyone knows each other

Technology doesn't have these limits. Things made of code can scale past human comprehension in seconds. And it changes it's users

Part of the ethics of software development is to carefully consider the ramifications of what you bring into the world.

The public can't make an informed choice, because they lack both the nuanced understanding of the tech, and every choice has a cognitive load. It's up to you to make it safe and healthy or to inform them of the consequences, and you can't just put up a 26 research papers on the psychological and solciological considerations for hitting a button... No one is going to read that.

You also can't have booby traps - anything a user can do inadvertently or accidentally shouldn't have serious consequences.

There's some room for debate, but it all comes down to this: you're responsible for how an average user is going to use your technology. You should do all you can to make it easy to use the tech safely, you should add covers over the buttons that do something with consequences, and things with deeper ramifications should only be available to power users who presumably have the technical knowledge to make an informed choice themselves.

So onto this situation. Say you make this button "sub to /c/_____ and all sister communities". That's not really a choice - it's like you go to McDonald's and order a burger, and they say "for the same price, I'll give you 3 additional burgers with different options". Some people would say no, but they wanted a thing and you offer them more of the thing. If they haven't tried them before, there's fomo - what if one of the other burgers is better? And it's not like they couldn't just stop eating.

The majority will accept 4 burgers, because they don't see the hidden consequences. There's no world where the average person sits down with 4 burgers and eats less than they would if they had 1 - it's human nature, studied and documented... Giving someone more food leads to them eating more, because we judge the amount we're eating in large part visually. Put it on a larger plate or pile it higher, and we underestimate how much we've eaten. Put it on a small plate, and we eat less.

Sure, there's people who understand this - those of us who've struggled with weight or food scarcity are either not going to accept the burgers, or we'll set 3 aside for later. There's people who might benefit from eating 4 burgers - someone who burns 10k calories a day needs that kind of intake (even though they'd be better off with more variety).

Good or bad, you've increased consumption based on how you've presented this choice. The outcome was a statistical certainty, but technically it was a choice. It's just a choice that every human would naturally answer the same way if they went in blind - do you want only the thing you asked for, or that plus more free stuff.

So if you make this a button, it'll overwhelm the single sub option. And there's a game theory aspect to this - I'd likely hit the button too, because individual choices here don't matter, it's a matter of speed and volume of users subbing and unsubbing