I think the problem is how final those review scores feel. Dan has already said that he won't update the review (for now, I'm sure he'll change his mind eventually) so Prey has now officially a 4 from the biggest review site for the rest of eternity. The review that he worked hard on won't make sense as soon as next week, and his real thoughts on the game won't get noticed because of it. It's just so short sighted.
IGN created reviews-in-progress to be more flexible with their review process, and the fact that Dan decided to go past it and give it this arbitrary score tells me he wanted to make some kind of statement. Out of frustration probably, he said he contacted Arkane, the issue was still there, he decided to go big to express taht frustration. I guess the problem too is how inconsistent this might feel compared to other reviews.
You could also argue that a reviewer has a responsability to know about the game outside of his narrow experience. Sure subjectivity is key, but you also have to realize that your experience might not be the one that everybody has, and you might even be in the extreme minority. So a score that feels very final and does not reflect the experience of 99% of the players, that'll feel irrelevant in less than a week, is a problem. But I'm sure Dan knew that, before he posted it he was preemptively descibing himself as a martyr on twitter.