The way I read that section is it is explicitly about the requirements on contributing… that’s the subheading, ‘contribute’ and makes those requirements unambiguous. It means that you cannot add any non-shareable code. Anything you contribute must be as permissive as the licenses given as examples.
There is I think a widespread practice for people new to FOSS licensing that a FOSS license means ‘you can do whatever you like’ with code under a FOSS license.
The image is of developers literally making stuff and then throwing it over the fence for anyone to come along and ‘steal’ as soon as they find it.
Generally, because monitoring and enforcement is so weak, this is the norm.
But, should one get caught using FOSS code without the correct attributions and so forth, it’s not really ‘theft’ anyway strictly speaking, it’s copyright infringement.
So, if you take some MIT code, you can’t just fork it and then distribute it under a new license. You still have to include the copyright notice and license text.
AFAIK there are not many licenses where you could argue that the license allows relicensing… maybe the public domain equivalent licenses are as near as you could get to a licensing regime that might imply that relicensing is allowed, but again AFAIK no FOSS license allows relicensing and the license follows the code around no matter where it goes.
Parity explicitly says it cannot be revoked, and does not mention something called ‘sublicensing’ either which might allow a little more flexibility downstream but no, once you have signed up for Parity, that’s basically the way your the main body of the code is going to stay BTL.
From the article you referenced:
Another favorite part about this license for me is that it allows others to license their own software in a more permissive license than Parity itself, so it’s not the same kind of “viral” as other copyleft licenses.
That isn’t the way I read it. My interpretation is more like:
…it allows others to license their contributions in a more permissive way.
Overall, the software license would still be Parity, but if you added some feature branch to the repo, that feature could be BSD or whatever, if you like - rather than having to be strict copyleft.