People are sorted into different classes in companies. You’ll never get a company to pay six figures for someone they consider a free intern - regardless of whether it’s in their economic interest.
And yet here they are, having lost that percentage of revenue because they had this mindset. I don't think she needed a six figure salary necessarily, but BF made exactly no attempt to do _anything_. Unlike an unpaid intern, McMahon could have kept doing this indefinitely, not just for three summer months.
But the larger point is that economically productive employees in their early career are almost always exploited, so it wouldn't have been hard for Buzzfeed to throw some relatively token amount towards her and have her be over the moon with it. Most college students would love to get $20k for doing something they were procrastinating and doing for free, especially if you package it with an ego-boosting title.