Employees are bound by contract. You don't need IP laws to prohibit them from sharing the code.

What if your code gets "snuck out", and you don't know who did it. Should it still be free to copy and distribute at that point?

Yes. The question is: if the code gets snuck out, do you really think IP laws will actually prevent people from distributing it?

Yeah, it's certainly working fine:

https://thepiratebay.se/torrent/7087027/Symantec_Norton_Anti...

https://thepiratebay.se/torrent/4149808/Windows_NT4_source_c...

https://thepiratebay.se/torrent/3497574/Windows_2000_source_...

"That which is not seen" is a great deal of other software that otherwise might be out there were there not laws against that sort of thing.

I'd like to see some evidence of that, please.

Well, the links you pointed to are for fairly old versions of Windows, and they are on a pirate site, rather than any old site that chooses to host them in the US. Is Mac OS there in its entirety? How about Oracle? DB2 from IBM?

So the fact that the source isn't there is proof that copyright prevented it? I may have a tiger repellent rock to sell you.

You need to provide evidence that the source of the Mac OS, Oracle or DB2 would be there if copyright didn't exist.

Look at the availability of Linux source code compared to Mac OS X. It's everywhere.

In terms of proof and evidence, no one can really "prove" anything without actually experiencing the counterfactual world. You can't prove that MacOS X code would not be everywhere, for instance, just as I can't prove that it would be.

The best we can do is look for evidence that suggests how things might go.