Actually they specifically plan to load astronauts before fueling begins. If they're in the capsule the launch escape system (LES)[1] can propel them away from any on-pad disaster; not so if they're still walking across the gantry.

That said, NASA has raised concerns about loading crew pre-fueling as part of the NASA-SpaceX Commercial Crew development contract (CCDev).

[1] Here's a video of the LES test for Dragon 2: https://www.youtube.com/watch?v=1_FXVjf46T8