Right!
So, I'm flatly not interested in any kind of Good or Law that you couldn't talk a self-interested dragon into having. But! This is much more than zero Law.
The benefit of trustworthiness is that you are perceived as trustworthy, and the benefit of being perceived as trustworthy is that you reap the benefits of trust. And the benefits of trust are huge; people who are able to trust each other, if they're right to trust each other, will always win against people who can't.
You can always do better for yourself by exploiting someone who gormlessly trusts you than you can by guilelessly extending the same trust, but that's wholly extractive - it makes one person richer and one person poorer. If you're a highwayman passing through, that deal probably seems pretty good, but while highwaymen are the richest kind of forager, foragers are not the richest kind of human, and if you reap without sowing, the harvest gets thinner each year. Two people together can do more than two free agents; even robbers would rather be part of a band that robs everyone else.
This is more than half of why people gladly suffer sovereigns, by the way; the threat of punishment for treachery is delicious if it'll also be applied to your allies, delicious enough to accept the indignity and humiliation of servility, hence why bandits and pirates and adventuring parties appoint captains and sub-captains instead of just walking in the same direction and figuring things out from there.
The incentive to extend trust - if only it can be made to work - is so powerful that everyone's thinking about it all of the time - very nearly as often as they're thinking of ways to trick and swindle each other, really - and so it's no surprise that even some of the stupidest people alive have found ways to make it work.
But every step in this chain is an important one: if you aren't getting the benefits of trustworthiness, there's no call to pay the costs, or if you can get the benefits without actually having the quality, again, there's no call to pay the costs.
And it is observed that men generally judge more by the eye than by the hand, because it belongs to everybody to see you, but to few to come in touch with you; everyone sees what you appear to be, few really know what you are, and those few dare not oppose themselves to the opinion of the many. [1]
And men are so simple, and so subject to present necessities, that he who seeks to deceive will always find someone who will allow himself to be deceived. Alexander the Sixth did nothing else but deceive men, nor ever thought of doing otherwise, and he always found victims; for there was never a man who had greater power in asserting, or who with greater oaths would affirm a thing, yet would observe it less; nevertheless his deceits always succeeded according to his wishes, because he well understood this side of mankind. [2]
1. The Prince, pg. 44
2. The Prince, pg. 43