Your "easily" is still considerably more difficult than producing weapons grade materials the old fashioned way, so how does it matter? The fuel salt in a molten salt reactor is the safest place for any materials that pose a proliferation threat. It is both thermally and radiologically very hot, and confined to a chemical processing hot cell or the reactor itself, which makes it rather difficult to walk off with. Little of the thorium ends up as Np-237 in the first place, and it doesn't stop there--the reactor will turn it into Pu-238 and so on.
The standard LFTR design does not have the facilities to separate the Np-237 which comes out of the fuel salt with along with UF6, and goes right back into the core. A thermal breeder using the thorium fuel cycle has a very small margin for neutron loss, and if the fissile is diverted, the reactor will stop. Extra care will need to be taken with machines configured to produce Pu-238, but even that poses a significant challenge for diversion, and similarly will not go unnoticed.
Furthermore, this is the machine which is capable of making every nation on earth energy independent, and ending essentially all resource conflict. Once a nation has that, there is little motivation to produce bombs and risk losing it. There is also the fact that reactors provide the only means of destroying weapons grade materials, and provide abundant energy as a byproduct. Obstructing nuclear energy prevents that from ever happening, and will pose a substantially greater risk.