This isn't about the SEC at all. Arresting on a subset of eventual charges is something that nearly all prosecutors do, and for a large set of very good strategic reasons.
I think you'd be hard-pressed to find many examples of high-profile cases where the original arrest was based on all possible charges.
What argument are you presenting instead? Because he was charged for 7 instances of security fraud we should assume that all 25 are securities fraud? Regardless of whether the SEC eventually charges all 25, they've clearly decided to differentiate them all on some metric (otherwise, they would've charged them all at the same time).