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

Note that a human doesn't "plug in" to the rest of the launch vehicle the way a satellite does.

I imagine they _would_ do these tests with the crew compartment attached and dummy human loads strapped in to the crew seats.

That is how Virgil I. "Gus" Grissom, Edward H. White II, and Roger B. Chaffee died.

https://en.wikipedia.org/wiki/Apollo_1

They have a launch escape system. Although even when it works the human cargo might need cigarettes and "shots of vodka to help them relax"

https://en.wikipedia.org/wiki/Soyuz_7K-ST_No._16L