Yes, completely different rules for motocycles vs. passenger vehicles. Motorcycles also don't have restraints, airbags, etc.
It can be done on the 4runner, it's just expensive and there are liability issues for a commercial operation. Not to mention the problems with smog controls and such... adding an extra tank can do strange things for the leak-test and overflow preventers that can cause your computer to throw error codes. I live in a county where emissions testing is required, so that would kill it for me.
Even with all these problems, if there were enough demand someone would develop a drop-in sub tank or else a replacement for the OEM tank that is larger. But the market just isn't there. People just don't use mid-sized SUV's to haul heavy trailers long distances, which is the typical application for larger tanks.
Personally, on first thought, I'd love a larger capacity. I make a trip a few times a year that is just slightly longer than my 4Runner's range. It's annoying to have to fill up twice each direction (once a few dozen miles after setting out, again mid-trip or on arrival).
On the other hand, there's a reason why passenger jets don't carry a full fuel load on short-hop flights. The extra weight would cause a loss of mileage during daily driving.
