They know that "outlaw secure encryption to stop inside traders" is going to be an even tougher sell than "outlaw secure encryption to stop murderers," so they try to make an illogical argument involving scary criminals instead of a stronger argument with not-scary criminals.
The white collar crime, yeah, some forms of it might benefit from encrypted communication. But again, where is the evidence? Is the tax evasion (non-institutionalized) skyrocketing?
The whole claim is just very suspicious. I think there is so much open data today that even with encrypted phones it's much harder to leave less trace than in the past.
The FBI doesn't give a shit as long as it's the anus of the average citizen being ravaged. They only care when it's against the 1%.