Until referees are independent from the league, this will always be a thing.
It's impossible for them to be unbiased because, as Draymond pointed out, and as I pointed out in the Heat series, they are employed by the league and the league has financial goals. People are paying money to attend and watch games on TV, and they want to see everyone play -- especially stars.
They could have fouled out every single Miami Heat player by the 3rd quarter, but they understand much like Draymond understands, that if you call their bluff, they're going to cave. The Heat can't beat the Celtics in a legitimate contest, but if they foul every play and the refs only call the obvious ones (or most of them), they might have a chance right?
I'm not sure why he said it out loud, though. He's just asking for it now as the league has to maintain the illusion of impartiality and accuracy. So he's not only calling their bluff through the actions in game, but explicitly stating it now.