With a Uranium-based reactor you need a) a source of enriched Uranium (see proliferation risks), b) inspectors to check that you're not post-processing fuel rods to extract Plutonium.

With Thorium if the operator country wants to extract the U-233 you think "maybe let them, they won't like it".

With classic pwr rods can't be used for weapons because of parasitic isotopes. At best you need to run the reactor in a specific way that would trigger global attention looking at power fluctuations. And still it'll not be easy afterwards. You can buy enriched uranium from others. With th- I'm not sure but don't you need enriched material anyway to kickstart it?