NASA has the entire history of US Spaceflight in its collective memory. It has many memories of accidents occurring during fueling that resulted in the loss of the spacecraft. It has no memories of a crew being saved by the crew abort systems because it has only happened once[1] and that was with a Russian rocket.
Thus, going by what they "know", NASA knows that rockets can blow up when they are being fueled and they know that SpaceX blew up one of their rockets while fueling it, but they do not know if a launch escape system would save astronauts in that situation. Therefore SpaceX's plan is "risky" because they are exposing the crew to a "known" danger and relying on an "unknown" system to protect them from that danger. It is this kind of thinking that keeps NASA moving slowly and carefully.
SpaceX has essentially said, "Trust us, if anything went wrong the crew escape system would save them." Unfortunately they don't have a launch pad they can blow up on purpose to demonstrate that for NASA (it would help if they could). The last time they blew up a rocket while fueling it, it took months to recondition the pad for launches again.
[1] In 1983, an escape rocket pulled the three cosmonauts of Soyuz T-10-1 to safety when its booster exploded on the launch pad. As of 2015, this is the only case where an abort system saved a crew from a launchpad accident. -- https://www.space.com/29260-how-spacecraft-launch-aborts-wor...