I'm not sure why you're surprised about this? The Deposito accounts are not fixed-interest accounts, nor do they claim to be. These are instant-access accounts where the interest rate fluctuates according to prevailing market conditions. Every flexible savings account I've ever come across is the same, except for some offering promotional interest rates for the first year to lure you in.
In this case, PostFinance gave advanced notice of their intention to change the interest rate, so you had time to make alternative investment decisions before being affected.
If you want the security of a fixed interest rate, you should look at
alternative products like medium term notes or money market investments.