Diamond City appears to have a strong anti-Synth sentiment, to say the least. For instance, shopkeepers will refuse to do business with you if you don't protest your humanity loudly enough, and then there's:
The guy who's about to gun down his brother because he suspects he's a synth, and then the guards gun him down and loudly declare "this is Diamond City, there are no synths here!" (or something roughly to that effect).
So it seems like a very bad place to be a synth. Yet Nick Valentine is one, runs a prominent business, and seems to be wholly tolerated and non-controversial. That seems out of character for a city that's already purged a race of nonhumans (ghouls) for...being nonhuman.
Is there any explanation given for the incongruity? I thought that when I got to chat with Valentine I'd get to ask him not just "you're a synth?" but also "how is it that the people in Diamond City haven't torn you limb from limb and shoved the pieces in a trash-compactor?", but that doesn't eventuate.
Are there any dialog options with other Diamond City NPC's that shed light on exactly how/why Nick Valentine came to be accepted in a city that seems to be heavily anti-synth (and generally anti-nonhuman)?