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?