Frames have to flex. Therefor they are riveted. The theory behind rivets is that they fill up the hole completely instead of relying on the squeeze of the cap and nut.
This is theoretically correct but debatable in terms of practicality in this case. Properly tightened bolts are more than adequate for his purpose and unless the bolts loosen there will be no movement between the parts being held together. I could not imagine the amount of abuse you would have to heap on a bolted frame to cause the bolts to fail.
