It's not a "nesting limit", it's a cooling-off period before the reply link is shown, which increases with nesting. It's supposed to discourage heated back-and-forth replies, which historically have low value. Replying a level up defeats the purpose.

PG, if you're reading: Instead of delaying the display of the reply link, it might be better to instead display the link, but during the cooling-off period make it go to a page explaining why the user can't post right away. (And maybe also make the link a different color?) Otherwise, it simply looks like a bug or artificial system limit.