How is this better than downgrading the error into a warning? You'd be able to build but would still know something was wrong-ish. Certainly a warning would be sufficient to chase down any bugs, and is very freeing compared to an error.
But some people behave like that. They think it is someone else's job to chase down bugs in their code after they have knowingly submitted code with defects rather than deal with it immediately.
Classifying something as an error rather than a warning makes zero difference to programmers who take care not to knowingly ship defects. It only makes a difference to people who were going to ignore it and let someone else take up their slack. And to be frank: why on earth would you want to accomodate them?
Basically, the point of no-unused-imports is to reduce compilation time at scale. In the C++ ecosystem, there are lots of redundant and unused #includes. This means that a lot of the bytes sent to the C++ compiler are just thrown away. If you can guarantee that the compiler will never need to look at a source file it does not need to, you can cut down on the compilation time. For small projects, this doesn't matter much. For large projects, it does. Go was designed for large projects.