There are a bunch of things going on here so bear with me.
1. The speed immediately before the curve is 80mph. The curve itself is rated for 80mph but the official speed limit is 50. Why the difference? Because rail companies take passenger comfort seriously and 80mph through that curve would require passengers wear seatbelts and might possibly cause slight travel sickness. As an aside trains generally start at Philly by accelerating with an open (full) throttle. When they reach 80mph, it's usually at the point in the journey where the train now needs to slow to 50mph to pass the curve. Supposedly the brakes weren't activated and throttle closed, possibly because the driver was distracted by having a rock thrown at him, but... WE DON'T KNOW THIS and the headline of this story is premature.
2.Both ATC and PTC do as you describe. They include mechanisms to monitor the speed of trains and slow them if they're speeding. PTC even includes a GPS element. ATC is older, creakier, but...
3. ATC was not installed on the section immediately North of Philly because, reportedly, Amtrak engineers at the time didn't believe any trains would actually reach 80mph before hitting that curve. This was probably true at the time.
4. In the last year, Amtrak has introduced new locomotives, including the one used for Amtrak 188. These locomotives are considerably more powerful than the "Meatballs" they replaced.
So, that's currently the thinking. The most likely scenario right now appears to be that the engineer was distracted by rocks being thrown at the train at the critical moment where he was supposed to close the throttle and engage the brakes. Because it was a newer, more powerful, locomotive than the safety systems there were originally designed for, the train was able to accelerate to 105mph during that distracted period. Because there were no ATC or PTC systems active in that area, the train wasn't stopped automatically.
That's the _most likely_ scenario. There are many other possibilities, including a software problem on the locomotive (which, depending on the nature of the bug) could have rendered PTC or ATC ineffectual given they rely upon the loco to, you know, respond to its commands. The latter is unlikely, but it hasn't been ruled out yet.
We should do what commonsense requires, the accident may or may not have been caused by a lack of ATC, but we do know now that there exists the possibility of speed related accidents in that area and need it to be addressed. In the mean time, we should wait for the NTSB to do its job.