I suspect he's trying to appear "tough on Russia" before the midterms after appearing to be on his knees giving Putin a hummer a few months ago.
Of course he could soft-peddle and back-channel this, not yank himself out of the agreement, but it doesn't fit his style when can leverage it for political gain (or to backfill after losing political face).
It's also not quite fair to blame Obama for "ignoring" this when (a) he didn't and (b) the NATO report confirming the violation was released just recently. Once again, political gain, fingerpointing and his usual bullshit.
Having said all that, is he right to accuse them and criticize them? Do I believe him this time? Probably, because when it comes to American interests I trust Russia less than I trust him.
And Europeans should care given this is all about missiles that can hit you all.
More a personal than a European opinion to be honest. I've never for a moment been scared about the prospect of a nuclear war. It would never happen. You can't win a war by suiciding yourself and ultimately every country wants to win the war, on whatever terms their war may be.
I put it in the same area of asteroids and gamma ray bursts. Sure it COULD happen, but the chances are so low that it's pointless to really think too much about it.