Kudos to the author, and hopefully they don't get sued as a result. This bullshit with corporations trying to cover up security vulnerabilities (rather than fix them) needs to stop. "Sign this NDA or we will send the FBI to arrest you because you found that our banking website's security was completely fucking broken and told us about it." Jesus fucking christ.
No one should independently contact a company about this type of issue without first obtaining competent legal advice. And I do mean competent advice; most lawyers are very technically illiterate and will not be sympathetic, let alone familiar with the relevant areas of law. The researcher is lucky that TradeKing believed their NDA trick was sufficient. Even if the case here is weak, and I wouldn't necessarily assume…
IANAL though.