No, it's not Linux being broken, it's you not understanding it.
There is no directory "/us". There is a directory "/mnt/us". If you are inside "/mnt", "cd us" will get you into that directory. "cd /mnt/us" will get you into that directory whereever you currently are. "cd mnt/us" will get you into that directory if you are currently in "/".
It's all very logical, but it requires a little bit of learning and understanding. If you're not up to that task, then the developer's forum might be the wrong place.
I'm sorry if you are offended by this (I realize that I'm stating things quite bluntly), but the purpose of this forum is not to help newbies find their way around. Of course we try to help each other in gaining new insights, but at least a minimum of effort is also expected from your side.
This forum is mostly about the internals of the Kindle, that's why it's called the "developer's forum". Before poking around with the internals of a Linux system (which the Kindle is), you are at least expected to understand the basics of the system.