I've run into a few older books that are available on Kindle from Amazon, but aren't available as ebooks from the above sources. I can see older books maybe not being available digitally, but if there's a Kindle version obviously that's not the case.
They all want you to sign up for their email lists. I prefer to keep my email free of that kind of stuff. Some of them have RSS feeds which I don't mind.
One of the advantages of getting the emails or following the feeds is to get discount offers. Another option for this is to do a web search for something like "oreilly coupons". This will usually turn up valid discount codes. I'm not sure if the companies like that, but they're the ones offering the discounts, so I figure it's ok.
It's too bad that Amazon doesn't sell DRM-free books. It's not ideal to have to go to multiple suppliers, and set up accounts on each, and deal with their differences. Some take PayPal, some don't. O'Reilly automatically sends my books to my Dropbox, the others don't. Maybe someday Amazon will see the light. Until then, they're losing my business.
Several of the sources offer access to "pre-release" versions. At first it seemed attractive to get access sooner, but I ended up preferring to wait to read the final version.
I've been looking forward to Tor (science fiction) going DRM-free, but the date has slipped from "July" to "summer", and now it's September. There's already been pressure by other publishers against Tor authors. [Update: I see on Amazon that at least some Tor books say: "At the publisher's request, this title is being sold without Digital Rights Management software (DRM) applied." I'm not sure why the Tor web-site still says "Coming in Summer 2012", maybe that's just referring to Tor's online store.]
Just to be clear, I prefer to buy my ebooks without DRM because it gives me more flexibility in where and how I can read them. I do not do it because I want to "pirate" them.