Heads up agreement?

[Magicmanu]

I witnessed an incident recently and would like to hear the opinions from this forum. In a 2/5NL cash game, four palyers at a full table saw the flop. The flop action resulted in two players getting all in, and two players each with a couple of hundred behind. Before the turn, one of the players with money offered to the other player with money to check it down. At that point one of the all in players went ballistic, accusing them of collusion and demanding that their hands be killed.

It seems to me that in a live game for all intents and purposes this was a decision between two heads up players. Moreover, if they really were colluding, they would not want to get to the river.

Any opinions about whether this is permissible or not?

Thanks.