Yes, ATVs tires are not DOT rated, so they are not considered legal for road use.
My concern is related: the ATV tires catastrophically failing at roadway speeds. Most ATVs are much lighter than a Samurai, so ATV tires installed on a Samurai will likely be at their max load limit. ATV tires are also much more aggressive than street tires, and I can see all those soft rubber ribs being ripped off at roadway speeds. It doesn't paint a pretty picture in my mind.
I run 31" Super Swamper Bias TSLs on one of my beater Samurais, and it is a great offroad tire. I also flat tow this Samurai to the trails, and they track well on the road too. The TSL Bias are 6-ply tires, with 4-ply sidewalls so they are overkill for a lightweight Samurai. However, that means if you run them on the stock narrow rims, you can lower the air to 5psi or even lower.
I am taking this rig out to a trail clean up run tomorrow. Love those TSLs!