Yes -- the FBI has no mission to protect the public good. Their job is not to defend ordinary Americans against cyber attacks. That's a different governmental agency, if such an agency even exists (I know there is a defensive part of NSA that does things like SELinux and hardening redhat, etc).

The job of the FBI is to detect and prosecute crimes under their jurisdiction. They've decided they can better do their jobs keeping the 0-day secret than disclosing it -- if it's kept secret, it makes it easier for criminals to commit more crimes for them to prosecute, and simultaneously gives them more investigative tools, win win.