To add to what everyone else said, it's also because mod creators are so fed up when pirates complain the mod doesn't work. I used to frequent a modding forum and you'd be surprised how many people beg for support only for everyone else to realize they pirated an outdated version or something.

They do that as a legal waiver. Mods typically do not add any code that adds in DRM, or check to make sure the DRM is functioning, except in the case of the game utilizing an online multiplayer function, which could attempt to be a prevention for cheaters.

Very likely it's to avoid getting into hot water with the publisher and also to try and mitigate support requests from people who installed 3rd party stuff that may or may not have caused the issue.

Usually it's just a legal ass-covering, but in this case, the project most likely depends on F4SE and the address library, which injects stuff at specific places in memory. Theoretically a cracked exe could work, but given something is likely changed from the original, all memory pointer addresses those two previous tools hook into and or override could be wrong. It's less of a piracy thing, and more of a "We don't want to troubleshoot things that may require us to do illegal things". Besides, the issue would have to be fixed in the original libraries, not the mod.

Downgrading should be possible on steam via steam console commands. I noticed large collections on Nexus usually tell you to use some sort of downgrader program but I've always just used the steam console instead and been fine.

It says in the article that it comes with a downgrader packaged in the installer. That said they're working with GoG, so the downgrader might not work for the steam edition.

Why would they do that. No they didn't. They wanted to push something official because of the show and generally don't bother not breaking mods with updates.

Its... A free mod. But if you are one of the 8 people that doesn't already own a copy of Fallout 4, go ahead and pirate it, they've made their money.