Many drivers disliked the more frequent warnings, in part because they didn't know how to stop them. "I now need to white knuckle death grip the wheel to keep away the nags," said one. This information is in the owner's manual, but drivers either didn't read it or had forgot that the wheel isn't grip sensitive, but instead detects turning motions (ie, steering). The new warning should thus make it clearer exactly how you can get rid of it.
Tesla already had a fairly rigorous Autopilot warning system, implemented in 2016 after investigators found that a driver's hands were off the wheel for several minutes before a crash. However, the warnings probably wouldn't have helped in the crash that killed engineer Walter Huang, as the NTSB found he had his hands on the wheel until just six seconds before the crash.
As Elon Musk's tweet above reflects, Tesla is trying to find a balance between the potential benefits of Autopilot and terrible human behavior. The system has proven multiple times that it can increase safety -- In one case, the onboard radar spotted a car that had slowed well ahead and applied the brakes before the driver could even see the resulting accident. On the other hand, some drivers assume that the Autopilot is literally that; one even took it so far as to leave the driver's seat at 40 mph on a highway.
Update: This story has been updated with more information on how Tesla decides how frequently to "nag" drivers to put their hands on the wheel.