If the software is being released as an API for other programmers to use (even internally), there should be some manner of documentation/user-guide provided. That might not be formal documentation. It might just be an email with some example code. But that sort of customer support is necessary since the code is the product in that case.
Otherwise, if the people involved (or the industry itself) are older, then the expectation of documentation is fair.
Before the advent of the agile development methodology, formal documentation was a key part of the process. More formal industries (medical, banking, government) or industries that move more slowly (manufacturing for example) still make use of documentation. Also, people who learned about software development back before agile might expect documentation to be par for the course.
But for almost everyone else, they should be familiar with modern software development practices that tend to eschew documentation. It is unreasonable to assume that the developer will produce documentation without prompting.
Though, as you say, it is pretty trivial to explicitly include that as a requirement. When in doubt, communicate.