Riot can't help when taxes go up. If the price of say, milk, goes up because sales tax increases (and that store has to pay taxes), do you complain that the store should take the hit and make up the difference for you? No, that'd be silly, because it's not the store's fault, it's just the government raising taxes.
Riot needs to pay those taxes based on what the governments have said so all they are doing is what any other seller of an item would do on an increase of taxes.
EDIT: Let me try to explain this better. The price of RP (I'm using $ because I'm American) is $5 for 650 or so (I think, been a while). If I had to pay 9% tax on that, the price for the RP hasn't changed. It's still $5 for that amount. I just now have to pay $5.45 to cover the $0.45 tax that Riot is paying to my state. Riot isn't getting the money, the state is.
Also, having had to calculate tax into the price for something to keep it at the same price while taking the hit, it's not an easy thing to do. It's much easier to have the customer pay a little more for tax.
Don't forget that they are giving bonus RP for a while because of this.