It's not about being tubeless. It's about wheel strength. Cast wheels are less dense than forged, so they flex more than forged wheels. Bias ply tires have LOT less sidewall flex than radials, so putting radials on a bias ply wheel overstresses it. Center Lines sre usually forged. Forged wheels marked "Bias ply only" wheels are made thinner & lighter than ones for radials so they're more likely to bend under the increased loads associated with radials. Also, if you use radials on a bias ply wheel with rim screws, you're risking tearing the aluminum because radials flex more than bias ply & Center Line doesn't want to be liable for the ensuing carnage...
Tommy