>
my understanding was that in the realm of source code "by default" you only have rights to use that source code through a license or contractNo, not at all, and this is a key difference between OSS licenses and EULAs. By default "you" (the entity in question, which may be an individual human or legal organization) have the right to use lawfully acquired source code for absolutely anything you wish. What copyright governs is distribution (either of the copyright works or any derivatives, except for exemptions made by Fair Use in places that have such a legal concept). So if I just put source code up for my program and make it freely available and nothing else, then by default you may download that and run it or hack it up or whatever else you want, just as if I gave you a book I wrote you'd be free to mark up the pages or tear some out and reorder it or whatever. You don't need the copyright holder's permission for any use of it.
What you do need permission for is to then share any of that on. So standard open source created a fair and thus very strong quid pro quo essentially: someone is offered additional rights beyond what they'd have by default via a license that requests they do some task in consideration related specifically to that copyrighted work (for some licenses it might only be indemnification from liability and maybe giving credit somewhere, for copyleft it means applying the license to any derivative works as well). There is no click through or "by using this you agree to..." there, your acquisition of the source is entirely separate from a later choice to distribute the source. No license is needed for use, but if you don't agree with the license and don't get another one then you have no right under copyright to distribute [1].
Anyway that's a simplified basis for the theory behind OSS. It's founded in the simple and straightforward application of copyright law and contracts, and it's fundamentally quite fair to all parties and has a very limited and directly related to the work aim. As the article says the further away one tries to get to that the more complex and iffy things can become and the easier it may be to include something that is legally challengeable. Copyright is open source's hammer, but not every aspect of technology is a nail.
----
1: Note that in practice this can at least theoretically get sticky if you're a programmer in a related field (as so many on HN are) rather then a random user, because having read the source code if you then down the road wrote something that looked the same it could be claimed it was derived which is then a huge pain to fight about. For non-high profile areas it's unlikely it'd ever come up, but the future is hard to predict in that area of life so many ethical or just cautious programmers would be careful about looking at source that wasn't open source.