Their current fee on Segwit Bitcoin is 0.0005BTC which is still in the range for normal priority speed.
Don't forget Binance is a centralized exchange, they're earn money through trading fee and withdrawal fee, if the mempool is still like this or become worse, I believe Binance will charge higher fee sooner or later.
shouldn't they be able to provide a priority-line for their exchange transactions? Yes, they would miss out on some of the high-fees they could otherwise rake in if they wouldn't prioritize their own transactions. But isn't that a price you should be willing to pay to maintain a stable and reliable monopoly-exchange such as Binance?
I don't think they will do that because there's no obligation for them to prioritize their customers, if someone complaint about slow transactions, they can just tell it's due to the current mempool condition. AFAIK they don't have a promotion about fast withdrawal among all centralized exchange too.