Legal and ethical aren't the same thing, though. I agree it's legal for your employer to monitor traffic on their network. But an ethical sysadmin would not facilitate their doing so (unless there were a fairly significant and unusual justification in context).

(Note: I would also never trust a company device or company network, and I keep my personal devices completely separate from the company network for this reason. But I consider this a workaround for a deplorable situation, rather than just the way things are.)

Personally I think that is too simplistic a position and the reality is more complex. Most people would agree that using this approach to spy on your employees to track their banking activity is unethical. Using MITM-SSL as a way to get visibility on certain APTs using products such as FireEye is controversial, but I don't personally believe to be unethical.

I would argue against such an approach if there are alternatives but if the organization's leaders were set on it I would engage with the process and make sure that it did not evolve into more unethical practices such as logging all traffic contents or the above banking example.

This is not a US discussion though, it's about the whole web.