Niantic was clear about the product of the labor: In exchange for swiping the PokeStop, you'd get the rewards. No one was ever told they'd get more than that, and no one had any reasonable expectation that they'd get more.
Expectations are often unreasonable from some perspective. It's not difficult to see why users are upset.
Exactly! Everyone thought that the exchange was them doing something in the game, and Niantic was giving them the rewards in the game, and no one had any reasonable expectation that Niantic would get more outside of the game. (After all, neither Blizzard or Square get anything when one completes quest objectives in their MMO.)
So obviously, now that Niantic is getting things outside the game its reasonable the people who did the work ask for something from that.