How common is it for employers to lay off employees for non-performance reasons

I see a lot of news regarding big corporations like Google, Microsoft, etc laying off employees due to 'strategic reasons', 'overhiring', etc.

I wanted to understand how common is it in the industry? Do startups and mid-sized companies do it often?

If this happens very often, is it then wrong from employees to treat employers instrumentally. That is, should we frown upon people who leave after a few weeks of joinig a company because they got a much better offer? Or they did not join the company whose offer they had accepted because they got a better offer?

Founders of HN, what do you feel about this?