I dont know where you read they are not street legal- The first place I looked says DOT approved (Summit Racing website) There are honestly no wheel police anyway- you can run any wheel you want and nobody would ever know the difference.
Weight limit exceeds these wheels? The 15x8 I looked at says 4,000 lbs rated- that is per wheel. So they are rated for a 16,000 lb 4 wheeled vehicle.
That being said, these are the "Nascar" style of wheels, and developed as an inexpensive racing wheel that is strong enough to take a beating. As a consequence- that means they are very heavy, and can degrade performance due to their weight.