Matthew 20.15
"Am I not allowed to do what what I choose with what belongs to me? Or are you envious because I am generous?"
The assertion goes: "Ah ha! See! God is sovereign, choosing to do what he will no matter what our ideas of fairness."
In the parable in Matthew 20, the master goes out and hires multiple groups of laborers, each of which gets the same wage no matter when during the day the master hired him. If the laborer worked all day he got a day's wage. If he worked the last two hours he got a day's wage. People almost always miss a couple of critical details: first, at the end, the all-day laborers aren't asking that the master pay the two-hour laborers less than them; they're asking they the master pay them more than he agreed on for a day's labor. As if that weren't enough, what shocks them more is not the sense of exclusiveness that we draw from the passage, but his radical, unfair inclusiveness. The master doesn't explain his actions, and he doesn't have to. He gave the full-day workers exactly what he told them he would. It's his business if he chooses to do the same with everyone. The Jewish audience was offended at the inclusiveness of Jesus' message. They were the people of the covenant, those YHWH entrusted with Torah; why should anyone else get the same reward?
Romans 9.13
"Jacob I loved
but I have hated Esau."
Romans 9.15
"I will have mercy on whom I have mercy,
and I will have compassion on whom I have compassion."
The assertion usually goes like this: "who are we to argue? If God seems unjust in electing one person over another, then we simply have to accept it. He's God. He can do what he wants."
The passage from Romans is part of a more extended argument by Paul wherein he covers several topics, one of which is Judaism's assumption that they have a special place in the Kingdom of God as God's covenant people, those to whom he entrusted Torah. Paul replies with a resounding "no", to which the response is predictable. They cry loudly and slowly, "that's not fair!" Romans 9 is the response to that. The inclusion of the Gentile on the same basis of faith is not an indication that anything was wrong; indeed it was the point from the beginning. Besides, as God arbitrarily elected Israel, so he has arbitrarily elected the Gentiles in addition now. This is the reality behind the scripture Paul quotes in vss. 13 and 15 (Mal 1.2-3 and Ex 33.19). Jacob was the younger brother. By right he had no claim to the birthright; he even came by it by foul means, but he was God's "elect" as it were. If you read through the rest of Romans 9, 10 and 11 you'll see that Paul maintains this assertion in a more extended way throughout. Yes, the Gentile is on a par with the Jew now, but fear not; God has not forgotten his covenant people.
Note this about both passages: while we want to use both to support exclusive predestination, using them to defend the justice of unconditional election and limited atonement, the actual argument each encounters is that God is unjust because he is more inclusive than the audience wanted him to be. It's the same point we find in Luke 15. The Pharisees are cast in the role of the elder brother, resentful of his father's inclusive, forgiving love. Also note that, while Paul brings the Gentiles into the mix, Jesus really doesn't. He's talking about "good" Jews versus "bad" Jews. He's telling the people who consider themselves "in" that they may very well be "last" while those on the "out" may very well be "first". The sinners and tax collectors are getting in before the really good Jews. Uh oh.
God's love is scandalous. It's unfair. In includes all the wrong people. It searches out and includes those who are most foreign to it. God is wasteful and extravagant (prodigal) in his love, giving it without scruple to the worst of us, saving and transforming every single person regardless of background as they kneel before the cross in faith and humility...and he cares nothing for how that makes the rest of us feel. We'll just have to trust him.