They said nothing but "this is how I felt, this is why I stopped using it". If I had a bad experience with some software, I too would uninstall it, even if I'm told "but you see, you didn't experience it correctly".
It's DRM, data caps and walled gardens that have convinced people otherwise. The next logical step is separating clients and servers altogether. Why should I pay for data if I'm only a consumer? In that case we might as well let youtube and netflix pay to deliver the content. And then it's back to cable TV again.
PS. You also have to remember that Spotify is 10 years old. Internet infrastructure and things like cloud offerings wasn't as good. It wasn't just a move to save bandwidth, but to actually be able to offer a good service (low latency, high throughput, for free). Which still isn't easy.
If I mistakenly believed you killed my mother and shot you in the heat of the moment, would I be entitled to my feeling?
I would feel it is justified to be miffed if they tried to do it in secret and it was determined that they did indeed try to hide or downplay this fact, but this is not in fact what happened. Their stance seemed to be "yeah, this is currently how it works, we know some customers are unhappy with the arrangement, so we'll eventually change it, but you can't currently turn uploads off." Thus, you are basing your indignation off your own ignorance, and not reality.
Full disclosure: Am long-time Spotify fan (hey, gotta be honest!)