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It is not. This is deeply unethical.

If a company takes its stock public, the shares sold generate cash which is held by the company selling them, and used to build the business. Nobody who bought them, including the insiders, is really forbidden from selling them (except insofar as insider trading laws apply), but if an insider sells shares this early it indicates a deep distrust by the seller of the underlying value of their own company. Doing so while letting outside investors spin is therefore unethical because it signals that the people who built the company are planning to abandon it and let the stock tank.

See also: lock-up period, which is not required during an IPO (news to me!) but the absence of a lock-up period sure seems like a red flag someone should have noticed. Although, I suppose, many people did.