PI on a hail mary - on each team - is kinda like holding on a QB scramble, if you go by the letter of the book then you could call it every single time.
When you put 4-5 receivers and 5+ DBs in the end zone and throw a ball at them, there is inevitably a lot of pushing and shoving and fighting for position. PI, by both teams, is inevitable and if you call it in a strict manner you'll never have an end to the game.
That's why refs only throw the flag when it's obvious and overwhelmingly done by one player/team to gain an extraordinary advantage. If a player is fairly open and a DB just bear hugs him to keep him from catching it, then it'd almost certainly get called. But what happened last night isn't that kind of play.
I get the idea that "a penalty on the first play of the game is a penalty on the last play" and generally agree, but the problem with the hail mary is that it's just a peculiar kind of play that you can't ref it like a normal play.