I don't want to speculate about the reasons why Grouspeak rose up the prices for Europeans. I don't understand VAT. I’ve just paid in Euros, although it’s almost 200 Czech Crowns (CZK) more expensive than the last year. It’s not nice. As for me, 200 CZK don’t mean a big problem. I earn quite well. But I know many people here who wavered over buying PM for 600 CZK (30 $). What will they do if they have to pay almost 800 CZK (30 €) instead now? Many of them will prefer being BM (= paying 0 $/€/CZK), I think. 800 CZK is too much for students and many other people in Czechia.
Or, as I know Czech people, many of them will "move" from EU so that they could pay in US Dollars. 200 CZK are worth doing this.