VPN users. If every VPN account has an unique monthly payment amount, a bank can check your transactions and know that it is you paying for it, because no one else would have a monthly payment matching that specific price. This example is a bit extreme because for the moment prices are not that unique. But if proton is creating, for example, 3 different prices, the pool of users that could own a certain account is reduced to one third just by looking at their subscription price.