Another little note is modders cannot charge for access to their mods. So, using Harmful Mechanic as an example (because you're here) he's allowed to ask for donations "for beer and other stuff" but if he were to charge for access to the mod including such examples as gatekeeping "early builds" behind a patreon subscription he would be in violation of his license agreement with Fractal Softworks and no longer be authorized to to create and distribute Mods.
Just a note here that the idea of charging for a mod is repugnant to me and I definitely wouldn't want that, even if it were something I could do under the license terms Alex has set out. I'm doing this for free so I can do it for fun and not have to worry about whether making worse work would bring in more money.
As to what would happen if a modder did start charging. I'm not a lawyer and while I doubt Alex would lawyer up (most likely a generic forum ban), I personally would consider their mod free-game to shamelessly rip assets from.
It actually wouldn't be - you can't violate someone's copyright just because they're jerks - and on top of that, you'd be exposing yourself legally, because the person in question could now claim
quantifiable monetary damages that your infringement caused. Besides, you wouldn't have to 'punish' a modder like this; Alex could C&D the mod, even if it were posted elsewhere (not really worth his time, but he'd have a legal basis to do so).
Lawyers still cost money, so it's unlikely anybody who cared about the tiny trickle of change that charging for a mod might bring in would sue you over it - but they'd have a legal basis for that if they wanted to. Never bet against some fraction of IP licenseholders being awful copyright trolls, no matter how small-change the stakes are. Look at the way Brad Wardell of Stardock tried to go after the Toys For Bob guys to bully them into handing over rights he hadn't bought to parts of the Star Control games - straightforwardly betting his pockets were deeper and he could mount a baseless suit longer than they could pay lawyers to assert their rights.
One of the reasons to be very leery of working on mods using preexisting intellectual property is that C&Ds are often sent out on a rolling basis determined by what in-house projects a license-holder has in the works - so your Star Trek mod might be ignored for years, even decades, and then someone at Paramount decides, 'let's make a 2D mobile Star Trek game, send out the due diligence team to make sure there's no infringing content out there that might be in competition with our game proposal' - and oops, modders making 2D Star Trek ship sprites all get hit with C&D notices.
The proposed game may never actually see the light of day, either. This is often done at the same stage of production where a project gets cancelled six months in and archived; it's something that gets handed off to recent grads at a big IP law firm.
(if this sounds
messed up, it is. This is one of the reasons I have CC licenses in my mod folders; to spell out
exactly what rights I'm relinquishing so you're
always free and clear for the authorized use. Traditional understanding of copyright would make even distributing free mods a potential copyright violation, albeit not one with much grounds for action; a CC license is
more generous in the right places, and lets you C&D someone redistributing mods with malware, for example.)