slonezp, I checked their websites and you are correct. After further investigating, it seems that the thickness of the aluminum, the expertise needed for welding, and the hull bracing determines if rivets or welding is suitable.
Airplanes have thin aluminum and welding aluminum makes it brittle at the weld site: it will crack during flexing. Thicker aluminum with "quality" welding is best for a "rugged" boat. Maybe Lund and Alumacraft companies don't have the expertise to create a durable welded hull with proper bracing to lessen hull flex.
The list of welded aluminum boats is quite long. Do a Google search on the subject.
Tracker makes riveted and welded jon boats. The riveted ones are the thinner aluminum, lighter weight models.
Thanks for the reply.