Software licensing already breaks many tacit rules that people normally take for granted. It's rude to copy someone else's creation without asking first - but if they license their code in a way that allows for copies to be created, it is no longer rude to copy that code without permission.
It is also rude to sell someone else's work without permission. But if they choose a software license without a non-commercial clause, it is no longer rude: and this implies that the absence of a feature in a license is a kind of approval of its opposite.
If you choose a license without an attribution clause in it, you are admitting, publicly, that you do not care about attribution - not that you require, nor that you forbid it, just that you are ambivalent. If somebody goes on to use your code without attribution, you are wrong to then point out that they have been "rude" to you, because you have already declared your indifference.
I'm not suggesting that unwritten rules are bad. I'm suggesting that trying to introduce unwritten rules to a system where written rules (i.e. licenses) already exist is a bad thing. Software licensing already sits at the intersection of legal and social obligations, because attribution is a feature with essentially no legal impact; treating a software license as a social contract is not a mistake.