I might be in the minority but the rules are pretty clear, and there must have been some poor miscommunication to fans to lead to this confusion.
1) If a receiver catches the ball and in doing so is NOT falling to the ground, he needs to ascertain possession with 2 feet (or 1 knee / shin etc) down and making a football move (tucking the ball / turning upfield / a 3rd step etc). This is what occurred with the Davante Adams play. He ascertains possession, and is then tackled to the ground. He is then down before the ball is ripped out. Therefore the ball stays with LA at the spot the ball is when his knee is down.
2) If a receiver is going to the ground as part of the process of the catch, then the rules are different. You still need the 2 feet (or 1 knee etc), but to gain possession you MUST survive the ground (aka the Calvin Johnson rule). This is the Buffalo play, as possession wasn't granted yet, the WRs knee being down does nothing. Indeed the defender having his hands on the ball does nothing either in terms of establishing possession. Both of them needed to survive the ground to gain possession, and as the defender rips the ball away at the end of play, that is why this is a clear INT as per the rules.
As always chat of "if its 50/50 the tie goes to the offense" is just misleading (it was the same with the fail mary narrative post game).
In response to Ivotuk above -
1. Yes. if you're going to the ground you need to survive the ground in the endzone for it to be a catch.
2. On the kickoff this is not the catch rule, this is covered by the rules of a recovery. This is determined by if control was gained by the player before a bodypart goes out of bounds. Which in the Pittsburg v Seattle game did occur, hence it was a TD.