The kind of thinking according to which this is unethical is the root of presenteeism and a butt-in-seats culture, which values time wasted over value created.
Paying someone by the hour while expecting results that can't be measured in terms of time spent is the actual unethical elephant in the room.
While technically working for less than the time stipulated in the contract is unethical, you could ask yourself: What's more beneficial to your employer and more in line with the meaning of the contract? Someone actually working for just 35 (instead of 40) hrs or someone just "doing their time" and not achieving anything during that time at all? My guess is, your employer would agree that it's the former. Unfortunately though, it's the latter that's quite common.
The only way to properly resolve this conflict is to pay people by the results they deliver not by the time they spend to achieve these results.
n.b.: As a contractor / freelancer the situation would be different. Ideally, in that case you aren't paid by the hour anyway but in terms of value-based pricing. If you are paid by your time, there's usually no fixed weekly amount. So, as long as you deliver good work you simply get paid the number of hours or days you've actually spent rather than some imaginary number.