What if it wasn't about the money? I wouldn't buy an ad alongside editorial content about my brand. Savvy media consumers tend to see that and think that the editorial content is actually paid content.
An intelligent brand manager has to weigh the positives and negatives of paid and earned media. In this case, in light of Saliterman's email, a brand manager would weigh the (seemingly high) possibility that the NRA would advertise alongside fearful messages from grieving families. Since Everytown knew that this was a possibility, the advertising alongside the editorial had the capacity to damage the brand and (perhaps more importantly) cost jobs.