Broadcom is notorious for demanding NDAs for their datasheets, completely pointlessly. I've also heard of companies demanding their parts be redacted on schematics if published, even more ridiculously, though as far as I know Broadcom isn't one of them (I've seen public schematics with Broadcom parts).
Still, from what I've heard of Broadcom specifically, they're pathologically secretive with their documentation unless some star customer dangles enough money in front of them. rpi.org certainly fits that description. So I don't really buy this excuse for rpi here.
In reality, I suspect it's more the fact that rpi.org was started by ex-Broadcom engineers and they brought their culture of secrecy with them.
Yet the chips weren't sold by any distributor. To even just get engineering samples from the manufacturer you had to sign this draconian NDA that covered everything from software to hardware docs even remotely connected to their part. Even after signing the NDA they wouldn't disclose full datasheet for e.g. what some registers did. For some things the comments in the Linux kernel turned out to be more informative than anything we got through official channels.
But if they're the only source of parts you have to use, you have to dance by their rules. Ruin the relationship by breaching the NDA and you might just as well bury the product you spent a year developing.
I have worked with much more mundane integrated circuits where the manufacturer required you to sign an NDA before getting any kind of design support from them. One of the terms of the NDA was not publishing the schematic of the device using their product. I don't see why Broadcom wouldn't do something similar for their SoCs.