Yep, it all comes down to self-control.How anyone can get fatter or out of shape doing physical activity baffles me. It's kind of like saying women don't find you attractive and it's their fault...no, maybe you're just friggin ugly. But, shouldn't physical exercise help you no matter what you are doing? Blaming an e-bike for you pushing maximum density seems a bit of a stretch. Especially when there is that handy, dandy little button to lower the assist.
If you sat on the couch all day and then bought and Ebike, which you then rode two three times a week at reasonable length ride, you should definitely lose weight.
If you rode and normal bike two of three times a week and then bought and Ebike, and you rode that the same amount, but in boost the whole time, I would think that you would gain some weight.
It's not brain science or rocket surgery, if you put in some more effort, you burn more calories, if you don't put in the effort, you won't. Same goes for food intake. If you eat more calories than you burn off, you will put on weight.
I put on a few kg last year as I was busy with renovations so wasn't able to get out and ride as much. Now I'm riding more again, I'm losing that weight (as well as gaining muscle and fitness).