Firstly, not just any FBO is a point of entry.
Which brings us to the second point, the entire reason for designated points of entry is so that the customs officials can be on site already. As in, assigned to that FBO. Now at times specialists have to be sent out. (Think exotic or rare animals or biological/agricultural products.) But if that happens, your freight, and maybe even you, are quarantined and your freight isn't going anywhere any time soon. Believe me.
Most important, and relevant in this context is the third issue. Which is the fact that arrivals are met by customs officials and passengers and cargo are always subject to the same inspections/regulations as they are at any commercial airport.
So the original question is valid, how are they handling customs at the scale they're hoping to achieve in a fashion any faster than anyone else?