I would like to say "serves them right" but these restrictions only hurt ordinary people, not the responsible governments.
But those affected are also those who are responsible for voting in their legislative assembly. That's actually pretty much how these things are supposed to work: if a country reneges on its agreements, its citizens will feel the effects, and they're the ones ultimately responsible for fixing the situation. I don't see any better alternatives.
Note: this issue isn't just about new EU states, but also citizens of existing states being arbitrarily excluded due to new laws that congress introduced in 2015.
// Edit: this is the official EU letter regarding the December 2015 US legislation:
http://www.niacaction.org/eu-respond...r-reciprocity/