If Fargo chooses to pay for a different engine over using Unigine for free, it doesn't necessarily mean that Unigine is a poor product. But people are going to perceive it that way. Even if Fargo goes out of his way to say that Unigine is quality software, people are still going to know it as that game engine Wasteland 2 wouldn't use for free.
So now one fallacy turned to three fallacies.
1. Just because some people get something for free doesn't make it worthless. Just because John Doe can go to bed with his wife doesn't mean it would cost me zero to do the same. Just 'cause Bob Smith can pirate a game for free doesn't mean this game is worth nothing. This was the original one, now yours.
2. Who are those "people" who'd perceive Unigine as crap? One hundred and seventy two gamers from RPG Codex? That's be a major loss to whoever decides on the engine in company XYZ. Unless those people have any say in what's next for other developers, their perception is meaningless. And most likely their memory will be short.
3. With all due respect but world does not turn around the Wasteland universe. Unigine won't be remembered as "the one not picked for W2". It will be forgotten by fans. Technical folks who decide on the tech purchase won't care what wasn't released using Unigine. They'll be interested in what Unigine enables. Whether it's a lot or not is a completely different story.
No one with any decency would make fool of them but likewise, no one with ethics would make such a one-way public offer. It was rude and imposing. You just don't declare something like that in public, regardless of your intentions. It can also put Fargo in a bad light. There are a lot of people who would bemoan a decision to reject it, no matter how kindly or well explained, because they are so disconnected from any aspect of game development, the only thing they would see is this: "This man asked us for our money because he couldn't do it with publishers so we gave it to him. Now he's turning down stuff that he could get for free and save more money for the game itself. WATS RONG WITHHIM", regardless of any reasoning Fargo might provide.
What's unethical in "we like this KS campaign, we'll give our engine away if they want to"? Stealing is unethical. Killing is pretty much unethical (unless you live in Florida where it's self defense). "Take it or else" is imposing, "take it, why didn't you make up your mind yet?!" is rude. It was just an option for inXile to explore. If they turn down the offer, so be it. It's not the end of the world for anyone.
As for the "not in public" - really? Where did Fargo start "thinking" about W2 KS campaign? Where does he shout news? How does he communicate with people? Twitter, public medium. Let's assume for a moment that Den donated to this campaign. Is his "hey, you can use this engine" any more unacceptable than "hey, I wan't an awesome Scorpitron" or "my rangers better have red boots, I donated, god damn it!"? I don't think so.
What people make of accepting/declining offer is in fact relevant. That's why I wouldn't expect any comments from Fargo before campaign is done. People do complain a lot about random stuff and not taking this piece of middleware would be viewed as wasting money by some for sure. But those people trusted Fargo in the first place with their money. Why would they suddenly question his decision and doubt in genuine explanation as to why they went for, say, UE? If you trust the guy and then just turn on your heel 180 deg, than there's something wrong with you, not with the man you put your trust in, is it?