The Taliban are banning the use of foreign currency, not only to circumvent efforts by UNDP, UNICEF, and other western organizations to inject foreign currency into the local economy, but also to shore up foreign exchange reserves needed for food and other imports.
This would be akin to Executive Order 6102 by FDR ordering privately held gold to be turned in and exchanged for US dollars, in this case foreign currency would be exchanged for Afghani.