Clever. Infuriating. Trollish. The dark arts. Depends on your perspective.
I'm going with hilarious.
So many rolls. Like a ninja! You can't see me.
And while it may inspire plenty of wailing and gnashing of teeth, the bottom line also remains that the roll back onto the pitch was a useful thing to do. The harsh reality is that just like players needing to go down under contact for most fouls to get called, players need to roll back onto the pitch to ensure that their team doesn't have to play on with 10 men for an overly (unfairly?) long period.
If a player sustains an injury in the field of play — and make no mistake, this was an actual injury, seeing as how Costa ended going to hospital for scans — and goes down within bounds, play is stopped and treatment is administered (see: Oviedo's injury). But if the player goes down with injury out of bounds, play continues. Which is exactly what was happening in this case. Costa rolling back onto the pitch forced referee Mike Jones to stop the match, during which time Loïc Rémy was able to get ready.
As bonus, the time spent on Costa's injury — which, had he not rolled back on, we would've had to play out with 10-men — was part of the minimum of 7 minutes added on at the end. And that, as we know, was most key.