I bought a kindle for my girlfriend quite a while ago, and have used their store many times (one of the early models, and she is an avid reader.)
Your assertions are factually incorrect with regards to amazon (no idea about other vendors.) This is not a case of it changing over time either, the below has been true since the introduction of the kindle, and remains true to this day.
1) Not all e-books are DRM restricted.
Amazon offers many books which are out of copyright for free (which has probably led to a significant savings over time for me, as she likes to read the classics, and I would otherwise end up buying these in paper form.) Small publishers and self published works are also in many cases DRM free. We have purchased several of these from amazon as well, so they obviously exist.
2) Books from sources other than amazon can be loaded onto the kindle.
We have purchased several self published books directly from the author and loaded them onto the kindle, so these also obviously exist. I will also point out that various libraries lend current e-books which work on the kindle (including our local library.) It is free to borrow these, although it is time limited as with any other book checked out from a library.
The below includes some speculation, so it is not necessarily factual... but is most likely correct:
I seriously doubt the companies you listed conspired with publishers to "keep all the savings for themselves." The far more likely scenario is that these companies entered into an agreement with publishers who owned the sole publication rights to works they wished to carry in their store, so that they could... you know... carry it in their store. I am sure that amazon wants to make money on these as well, but I can hardly fault them for that (I also do not work unless I can make money from that work.)
One of the conditions was likely that they include DRM, and so they did. I am not obligated to purchase these if I do not wish to, and could in fact only purchase DRM free books for use with the kindle (from amazon or another store.) This would limit the selection of modern books, but it could be done (many could even be borrowed from the library... if we wanted to wait in line, as with any other popular physical book.)
There are some complaints to make here, but they are best leveled against the appropriate party. In this case publishers who are watching their industry collapse, and do not wish to embrace the changes our modern age brings. I do not believe amazon deserves this criticism however, and in fact think they deserve much credit for essentially creating the market.
I have no affiliation with amazon other than as a customer, and do not own their stock (I probably would, but their current valuation is hard to swallow.)