One problem with regulation is that regulators often have substantial latitude to choose the stringency and targets of their enforcement efforts. This opens the door for businesses and politicians to influence that enforcement.
A recent paper by Maria Correia of the London Business School finds exactly this effect at the SEC. Correia writes:
I examine whether firms and executives with long-term political connections through contributions and lobbying incur lower costs from the enforcement actions by the Securities and Exchange Commission (SEC). I find that politically connected firms on average are less likely to be involved in SEC enforcement actions and face lower penalties if they are prosecuted by the SEC. Contributions to politicians in a strong position to put pressure on the SEC are more effective than others at reducing the probability of enforcement and penalties imposed by an enforcement action. Moreover, the amounts paid to lobbyists with prior employment links to the SEC, and the amounts spent on lobbying the SEC directly, are more effective than other lobbying expenditures at reducing enforcement costs faced by firms.
So SEC enforcement does not necessarilly target the firms whose behavior is "worst" but instead firms whose political connections are weakest.