Despite the lower price of the silver, the "tax" is a static fiat-based % so basically more people are donating the same %. All the people that were donating 1.5% if holding a balance over $380 would still be donating it (and the same $amount) if the threshold were $360 (silver price drops), more people would be donating since now everyone between $360 and $380 would also be donating a 1.5% in addition.
If the donation was in silver then it might even itself out, but you'd have to look at a distribution curve of income to know for sure.
From the twitter posts:
> Individuals who fall just below the threshold one year due the chance price of silver suddenly avoid a 2.5% gvt tax. Those people then increase their private donations to charity (since they have more disposable income + islam calls for alms-giving).
Or in clearer terms:
> ↑silver price → ↑tax threshold → ↓taxes → ↑charity → ↑terrorist financing → ↑terrorist attacks!
According to the post if the price of silver is higher, the tax threshold increases and more people have extra money to support whatever organization they want.