This is typical of Hetzner, if a product SKU is losing money they very quickly make changes, even going as far as to discontinue the product entirely (eg. GPU servers). They definitely don't seem to be a fan of loss leaders.
I'm guessing somehow the traffic usage patterns of their USA customers was very different to their EU counterparts, or the cost of expanding network capacity was a lot higher than anticipated.
It's a bit of a shock for sure but it seems this model is a big part of how they can maintain their slim margins.