Every hardware manufacturer can be instructed/bribed/forced to add backdoors to their hardware by their own government, hence the necessary push for open drivers/firmware (Broadcom itself, just to name one, has had strong ties with the US govt for a long time).
I can imagine a meeting in which some high rank officer says "Here's our backdoor blob, you merge this to all your chipsets firmware, so when necessary we can selectively either shut off Ethernet chips or have them relay information elsewhere as instructed through magic packets, which of course won't be noticed because the leds won't blink and any other chipset seeing these packets will comply as well letting them pass through without allowing any form of sniffing or telling the system administrator (1)". I can't imagine any manufacturer risking their business by replying "nope, we won't comply"; they will jump when commanded to do so and if caught the standard reply will be "we were forced" or "they say it's for national security, you know, to catch those evil terrorists!".
(1) it may seem absurd, sort of sci-fi, but having access to the underlying hardware and its firmware would make it not that hard to do. In that case, the only way to safely analyze network traffic would require very fast logic analyzers that wouldn't use any dedicated network chipsets.
The point is: security through obscurity usually doesn't work well, unless the untrusted party is the hardware maker itself, or whoever decides what they put in the hardware; in that case security through obscurity becomes a lot more about obscurity than security, which makes it near 100% effective.