[luckyno13]

Only an assumption here, but I would say its due to what to do with the waste. If the waste is useless, then we have to have a place to put it. If it has another use, then obviously we dont have to stick it in a hole somewhere.

Again, totally non-researched, off the top of my head assumption.

[seanflyon]

Thorium reactors breed U-233, but not as a waste product. The U-233 is consumed to generate power. I'm not sure what the U-233 produces when it undergoes fission but I don't think the result is weaponizable.