What IP did they steal. Please be specific.

It just isn't. You're allowed to make products that compete with products already on the market, even if you've worked for the company making that product. Unless there's specific IP that was stolen, I don't see how this is in any way unethical or shady. They hired you to develop that product, and you did. Nowhere in that contract is anything saying you can never use what you learned elsewhere. They'd need to pay you significantly more than what they paid you (which they won't). And there are great reasons why non-competes are basically invalid in most places.

Strange logic. Would you consider it unethical for former Google employees to start their own search engine? Or to quit and go work for Bing? Assuming they aren't stealing code this happens in every industry. Can former Ford employees not work for Chevy? I don't see the difference.

> don't spin-off that company using corporate knowledge

That is a very common way to do a spin off. Usually it involves some patents or non public knowledge and you make an agreement with the original company for it. But when it is all open there is nothing, legal or moral (in my opinion), from stopping you from making your own spinoff. Companies don't care about you, you shouldn't care about them.

> don't spin-off that company using corporate knowledge,

If everyone followed this rule there would be no Fairchild and no Intel.

I don't understand at all why this would be unethical. Maybe I'm working for a company making a product I have expertise for and I'm passionate about, but they're making decisions that I feel make it worse for users unnecessarily. I leave and start my own competing product to do it my own way that I consider to be the better way. This isn't unethical. This is the nature of all competition everywhere going back as far as anybody can remember. I've stolen nothing at all, so I'm not sure on what basis it would be unethical.

And if it's a topic or industry I have expertise and knowledge of, why would I throw that all away just because I worked for a company where I used that expertise and knowledge? That's basically implying that we're all "ethically" subject to non-competes for the rest of our lives after working for a company making a particular kind of product.

That's clearly and absolutely "inelegant", to use your phrasing.

Companies want you hire you and for you to bring your existing knowledge, skills, and understanding to build a product.

Then they want you to not use the knowledge, skills, and understanding to build a product when you aren't working for them.

This is hypocritical and self-contradictory. They require you to do the very thing they don't want you to do, just as long as it's in their favor.

It's neither illegal nor unethical to open a bake shop after working for a bake shop, or even a single type of cookie shop, as long as it's not a solen private recipe.

If the intern was under a no-compete, or stole any IP, this would be known and there would be nothing to talk about.

This is ridiculous. There are no rules, not even ethical ones, preventing you from starting a competing company. Especially since generally we don't start companies "putting our effort into destroying the jobs of our former co-workers". That's an absurd take.

I prefer "treat others as they would treat you" as a moral principle - or slightly better than they would treat you. How do you think your company would treat you, if they found a developer that did a better version of whatever you're doing? If you found a better way to do whatever your company is doing, the answer should be the same.