As if being banned would stop people from arguing that their ban was unjust.
We had folks PM us for months and months after a permaban was put into place to try to argue their innocence or that the ban was unfair. After a couple of exchanges and with nothing new to add, we'd just ignore them and let them tire themselves out arguing.
But there is a bit of truth to what you're saying, but not quite in the way you're seeing it. Folks like the ones on here who post in this manner typically don't only begin this once a ban is put in, but their normal posting style was already like this before any bans were entered. And the fact that they often ping around the board posting like this, which largely isn't conducive to great discussion, does often end up playing a role in that banning them doesn't take much away from the greater board.
As I said previously, as mods we often viewed the board as a kind of online bar and if a particularly poster could best be described as the obnoxious drunk who annoys everyone at the bar, then that certainly factored into the decision-making as to when to be done with that poster.