This is exclusive to this year. Every other year, it's not just NCAA players that can come late in the season/playoffs. It's players out of Europe, too. If this was last year, Romanov would be playing.
For some reason, they decided not to allow teams to do that this year.
Ian Mitchell for Chicago and Scott Perunovich for St. Louis are both in the same boat as Romanov with Habs, and they're both coming out of the NCAA.
So it's not a case of NCAA players can join and players out of Europe can't. Every other year, they both can. This year, neither can.
It may end up changing as it's not set in stone for this year yet. The NHLPA needs to approve. I imagine they will because they probably want to avoid any more hiccups or delays to this process.