Starling has become the first British bank to introduce negative interest rates for personal account customers, though only those who hold high balances in euros will be affected … lucky I don’t have a high balance in my Starling account 
They’re just doing what a lot of other banks outside the UK do. After all that’s the way today banking system works, as unless ECB raises interest again to anything >0%.
After all you (bank) cannot lose money with negative interests. Somebody else has to cover.