Each time, they pointed me to a part of the agreement that says you need to have "original content" that isn't primarily just ads. In each case, I know the content was original (I created it: mostly articles about DIY electronics, 3D printing, etc), there were no other ads on the sites other than a few (2 or 3 per page) affiliate links where I mentioned products that I had incorporated into my own DIY projects.
I wasn't able to get any further explanation from Amazon in these cases, so I don't even know why they kept shutting down my accounts - and I guess that's what concerns me the most. Even if they hadn't shut me down when they did, the fact remains that they could, at any point in time, shut down or significantly alter the terms of our "agreement" at their (seemingly arbitrary) will.