If you really want to know the technical stuff behind this, you could find this read interesting: https://golang.org/pkg/html/template/
Basically the template system that Hugo uses is context aware and doesn’t trust stuff that is parametrized. That’s why the safe functions exist, to tell the template explicitly “you can trust this”. However, I found that in some contexts even the result of the safe functions is filtered. I don’t think there’s a way to bypass this. However if you hardcode the attribute it is rendered as it was written (because it is trusted).