Letang clearly shoved Burmistrov from behind on the play, picking up a two-minute minor for boarding.
Despite the assurances of the Pittsburgh-based broadcast crew there, Letang was indeed suspended and it's certainly understandable within the rules. NHL discipline chief Brendan Shanahan explained his decision in a video posted Tuesday afternoon, saying that Letang delivered the check with force despite realizing that he was hitting Burmistrov in the numbers.
Shanahan also noted that Letang's prior history, which includes a fine for a similar hit, played into the decision. Here's the full explanation video from the NHL: