The new Nexus devices have USB Type-C ports. This is a new port that many of us don't have cables for, any most of our chargers don't support a standard USB Type-C cable, instead we need a USB Type-A to a Type-C cable to use our existing chargers and the ports on our laptops. With the Nexus 6P, Google did include a USB Type-A to Type-C connector, however the Nexus 5X does not. Google does sell one on the Google Store for $20, which is a pretty hefty price compared to the ones available on Amazon.
Yesterday, a person responding to a review on Amazon, claiming to be a Google Engineer informed everyone why the cables that come with the Nexus 5X or 6P are capable of 5V or 3A, while third-party ones are not. And it's mainly due to safety. The Google Engineer is Benson Leung who claims to have worked on both the Pixel and Nexus projects at Google. In particular, he worked on the USB Type-C connections on those devices.
In short, the reason for this is because of certification. For those unaware, USB Type-A ports as well as cables that can charge at 3A have an identifier resistor which is embedded in the cables which then communicate with the phone to charge at 3A. We don't have to tell you, but that's quite dangerous. So the cable that comes with your Nexus 5X or 6P (the cable with USB Type-C on both ends) is capable of 3A because of the way both the cable and the ports were engineered by Google, Huawei and LG. Leung also noted that none of the USB Type-A ports that are being sold on Amazon and elsewhere are not rated to support 3A. And those that are claiming to be rated at 3A are not in compliance and could severely damage your device, and whatever you have it plugged into. Whether that's a PC, a wall charger, or even a car charger. So, fair warning here, be careful with the USB cables you buy for your new Nexus 5X or 6P.