Sim boxes and etc aren't useful for terrorism, or at least you not anywhere near this many.

You only need this many for bulk messaging/calling. Legitimate bulk messaging/calling would be going through sip providers and SMS aggregators and/or interconnection with carriers at the kind of volumes you'd have this many sim boxes for. So it's got to be fraud/abuse of some sort. Probably selling bulk sms/calling to users that can't or don't want to use legitimate providers.

Again, I don’t know what the Secret Service isn’t saying, but I do know that failures of imagination have led to unpleasant surprises in the past. I’ll be very curious to learn more. Hopefully the details will come out.