The manufacturers do this to get around certain regulations, usually flotation. This is common in jon boats such as duck boats as they are going to be used in shallow water and don't really need flotation, thus flotation wastes space and is somewhat of a downside to say a guy duck hunting swamps running a mud motor to some degree. Check out prodrive boats, perfect example. I doubt you would have any issue and you do not have to start a golf ball retrieval business. However, if you plan on using your jon on lake michigan or are thinking about crossing the atlantic in it, then I would say add foam or get a model with flotation. Bottom line is do you need flotation for what you are using it for? If its not a major concern its not a major concern, if it is there are plenty of manufacturers making models with approved flotation.
What manufacturer/models are you looking at? Something else to consider is that often the boats that are built as commercial use only for this purpose are also built extremely beefy, to withstand running over trees and logs, through mud, etc. etc. So if you want a very well built boat you may want to go that route and add your own flotation if you desire. Its easier to add foam than ribs etc.
