Calvinists are notorious for asking the unsuspecting believer, “Why did you believe in Christ and someone else does not; are you smarter, or more praiseworthy in some way?” I asked this question more times than I can remember as a young Calvinist.
If someone disagreed with me, my presumption was that they must not really understand my perspective. So, instead of attempting to listen and objectively evaluate their arguments, I focused on restating my case more clearly, confidently, and dogmatically. If I did not fully understand what they were saying I would often label and dismiss them instead of taking the time to fully evaluate their point of view.
Piper is free to have and express his opinion about Arminians and exegesis, but it would be helpful if he would at least reveal which Arminian Bible scholars he has read and found wanting and why. Instead his response was merely dismissive and ought not to be taken very seriously—unless one takes whatever Piper says seriously just because Piper says it.
The concept of love (particularly how God loves) is a bit different on Calvinism. On Calvinism God loves the elect. Jesus died and rose again, for the few elect. This means that by definition, God as described by the consistent Calvinist is not omnibenevolent. Of course, a Calvinist could redefine the word “love” as it relates to God and claim that it is “loving” for God to allow those he loves to go to hell when they simply could not choose otherwise. In fact, this is the approach that many Calvinists take.
Even the idea of believing refers to individuals since only individuals can exercise faith; I do not see anything in the passage that would indicate that this “all” is restricted to something like all people groups, including some who can be saved. I do not think the passage even hints that this “all” is different from “all” including each person previously spoken of as under sin; of which “there is no distinction.” This understanding corresponds precisely with its usage before and in the following verses.