The level of cheapskatery demonstrated when it comes to transporting things on roofs goes beyond pennypinching. It shows lack of concern for the potential damage to either lives or other people’s property.
A hard, somewhat rotund object such as a kayak flying off a car roof is likely to travel faster and farther than a floppy mattress tied to a roof. Maybe you should compare throwing a football vs throwing a tortilla!
Use a real rack meant for kayaks.
If I’m on a motorcycle I don’t want to hit a mattress or a Christmas tree or a kayak traveling highway speeds. I don’t want to even be hit by a turkey flying low crossing the highway that I have dodged more than once. There are enough dangers that are outside of mans control like white tail deer around here that kill a few people every year. We don’t need to add more with stupidity.
I had a guy pull over and ream me out last summer when I was cutting my grass telling me that some of my clippings had blown on the road and could kill a motorcyclist. I told him I understood his concern but his car parked where it was with traffic going 50MPH might kill someone quicker and the tiny amount of my grass on the road if it can’t be rode over safely the person doesn’t belong on a bike. By the time he left it had all blown off the road. That was his thing he had issues with and I respect him having his opinion. I’m the guy that if I do get grass on the road I go out with the mower and blow it off before I finish.
Around here I see a round bail of hay on the road from time to time. Farmers trying to save time stack 22 of them on a hay wagon made for 18 and they bounce off. They have no way of putting them back on so if they see it they roll it to the side if not there it is a 5’ high solid block in the middle of the road.
Looks squashed
What is the code section that says its violates Federal law???
No you can loot and walk away.
Amazon sells a blow up rack for about 60 bucks. Packs to the size of 2 footballs end to end. Eliminating any roof scratches or movement. I try to not drive over 80 in high winds. 
Rack is awsume I have ( aside from local trips) two trips of 200 plus miles Carring a 16.5 on one trip and a 17 on another. Two straps across the belly and one each bow and stern ( included with kit) can’t speak highly enough, all your problems go away
Google is your friend. There’s all types of laws for transportation.
Your biggest problem here is that you didn’t spend $$$$$ for a rack and you have a perfectly good solution that works for you. I’d add foam blocks to keep my roof from getting scratched, but that’s me.
One of the worst ideas I have seen in awhile.