the WTBTS did not charge interest on the loans in none of their 239 countries and territories. Furthermore this arrangement is implemented in 239 countries and not in the US only so it has nothing to do with the IRS or US tax exempt status. If that was the case they would only change arrangements in the US.
anyway this is one of the best arrangements the WTBTS did come up with.
Before this new arrangement my congregation was donating about 125 dollars every month for Kingdom Hall fund but from now on this will be U$1145 dollars. And the other congregation in our Kingdom Hall will donate even more every single month U$1370