Insider trades are allowed through 10b5-1 plans. Basically, a corporate executive can make and register a plan to sell shares, and trades under the plan aren't subject to insider trading rules. This loophole reduces the effectiveness of insider trading regulations since the executive can cancel the trade for any reason, including material information.

If you invest in specific companies, always check the SEC filings for insider trades - they can be indicative of management's expectations for the company (though they can also be misleading for various reasons).