Binance want more and more money, as usual. And they want to push you to use BUSD
That might be the case and you understand how Business people think, they will try to take advantage of every situation to better their own and I am guessing that Binance might do that too or just trying to catch up with the fees from other exchange because I know that I was charged 1USDT transferring some USDT from one other exchange to BInance some time ago.