It sounds like you may have not read — or overlooked — this point in the article:

> "The airlines are intentionally selling that space to the person in front of you and to you, so it's just incredibly disingenuous of the airline," he said. "I understand why they like leaning. It's more of an airline issue because they're cramming seats so close together and pitting passengers against each other.”

The button’s there so that passengers buying the ticket feel like they have additional room. And while that’s somewhat true, that passenger is taking space away from the passenger behind.

So there is this shared space, and passengers are left to fend for themselves to determine who has rights to it.