See here for one example [0]. The SEC does have rules against it, so it's not entirely on a case-by-case basis [1]. It's just the SEC isn't supposed to write law.
[0] https://www.bloomberg.com/view/articles/2014-12-11/whats-nex...
It isn't really.
https://www.sec.gov/news/testimony/2011/ts120111rsk.htm
> There is no express statutory definition of the offense of insider trading in securities.3 The SEC prosecutes insider trading under the general antifraud provisions of the Federal securities laws, most commonly Section 10(b) of the Securities Exchange Act of 1934 (“Exchange Act”) and Rule 10b-5, a broad anti-fraud rule promulgated by the SEC under Section 10(b). Section 10(b) declares it unlawful “[t]o use or employ, in connection with the purchase or sale of any security . . . any manipulative or deceptive device or contrivance in contravention of such rules and regulations as the Commission may prescribe as necessary or appropriate in the public interest or for the protection of investors.”4 Rule 10b-5 broadly prohibits fraud and deception in connection with the purchase and sale of securities. As the Supreme Court has stated, “Section 10(b) and Rule 10b-5 prohibit all fraudulent schemes in connection with the purchase or sale of securities, whether the artifices employed involve a garden type variety of fraud, or present a unique form of deception,” because “[n]ovel or atypical methods should not provide immunity from the securities laws.”5
Congress wrote some very broad anti-fraud laws, never amended them, and left it up to the SEC to exercise its discretion on what qualified as such practices.
And it doesn't write the law, it writes regulation. Which, on this specific subject, it absolutely is supposed to under the law [0] (and if the law didn't specifically authorize the regulations and criminalize violations of them, the regulations wouldn't support criminal prosecution.)
[0] 15 USC § 78j, particularly subsection (b)