Yeah, I agree there are some 'hard to explain' things about this. At the end of the day it's all down to safety in some manner or form, and I guess they have to come up with some text which covers as many people as possible.
The one thing I don't see on many sites is about rear visibility when towing boats. Reading through all these caravan guides etc... they all talk about using wing mirror extenders to enable you to see behind, but I haven't seen anything like this for boats.
Watching people tow their boats around here quite a bit, there are many people who cannot see anything what's going on behind them, there are also quite a few people who don't even bother with tail boards with lights etc... too
Just to be on the safe side when we got my boat back from Hull, I borrowed one of the 3.5 ton flatbed trucks from work, which had plenty of power, was nice and wide, and seemed to tow the boat very well, although the overall length of truck and boat together was pretty large which made general handling a pretty good challenge.
I see what you mean about the width of the 'load' and the 'trailer', there is a significant difference between the two. I wonder how many people just throw any trailer on the back of their cars and just drive off without even thinking about the laws - probably not even worth thinking about