According to Tesla's Supercharger website, "Superchargers deliver energy rapidly, and gradually slow down as the battery fills. Your vehicle automatically alerts you when it has enough energy to continue the trip and with the extensive network of Superchargers along popular routes, charging above 80% isn't typically necessary." Now it's not just unnecessary, in some instances it won't be allowed. As you've probably already figured out, this change is intended to reduce wait times at these busiest of Supercharging stations.
"When combined with the recently released On-Route Battery Warmup feature and V2 Supercharger upgrades (to 150 kW), we expect 80% SOC Limit enforcement to result in a 34% improvement in throughput at our busiest Supercharging locations – creating a better, more efficient [Supercharging] experience for our owners," according to the memo.
If you're a Tesla owner and worried that this limitation will adversely impact your ability to drive long distances, there's apparently a way around the charging cap. Owners who are routed to a Supercharger station through Tesla's trip planner won't be subject to an 80% charge.