I've got a pair of ZTR Alpines, and run them with non-UST Rocket Ron Evo's - the advantage of this combo is the lightness of the setup which aids acceleration and responsiveness.
I've never used UST tyres, but would imagine they're made a little 'tighter' to ensure a good seal on the rim (!?) as well as having stronger sidewalls. As said above the ZTR rims are designed to run non-UST tyres though, so that's surely the way to go.
As far as running even UST tyres without sealant ... that doesn't sound like such a smart idea, because if you run over a thorn or somesuch you'd have to fit a tube anyway!? Then how would you repair the hole in the tyre?