The reasons I always figured about that were...
1. He doesn't want Leia or Han accidentally shot.
2. He could use Chewie's life later on as a bargaining chip to coerce Leia to work against the Rebellion and that's more valuable than a couple of stormies.
3. Firing blasters inside the chamber might have been dangerous.
4. He wants everyone alive including Chewie to draw Luke out.
It's a cool character moment for Fett that he's side stepping Vader's authority by taking matters into his own hands and Vader ain't havin' it.