They were doing what most players do -- getting away with as much as they can inside the current rules. A defender will interfere with a receiver just enough not to get a flag. An offensive lineman will hold just enough not to get a flag (sometimes they misjudge and get caught).
This is no different. And there is no way a ref can verify if a player is really injured even if the ref was a trained physician. There is no way to verify a cramp.
Under current NFL rules when the trainers have to come on the field, they must leave for the next play. They can return anytime after that (unless they have a concussion and then they need a series of approvals and that never happens the same game).
If you want to discourage it, change the rule so the player who is injured cannot re-enter the playing field on the current drive. If they admit to it, they can be disiplined. Without the admission, there is nothing that can be done.
As a tactic it worked. St. Louis' drive stalled because the Giants, with the break, were able to adjust their defence.
