Im not sure why you say that. I re-watched that play and Beli was there with a hand up in his face. Fouling the player would be worse.
Because he needed to be 2-3 feet further out than he was. Not standing a foot or so behind the line then having to extend out. As soon as the switch was occurring on the screen, he needed to quickly step out to be ON the line or even just past it. Doing so would have put the shooter in the position where he would have had to drive past him -- inside the 3 point line -- or force up a step back 3 from much further out.
The KINGS have a habit of not committing to the three point shooter in an obvious 3 point situation. They hedge and allow too much space, often being too far inside the 3 pt line. Pressing and crowding the shooter w/o fouling can be done. The opposition does it to the KINGS all the time. Belly just needed to be more aggressive in getting to the spot first. And he easily could have. But he didn't.
What he did was fine, if he had done it several feet further out from where he was. He collapsed under the line way too far.