In my opinion, the only way to prevent Amazon exclusivity (if you're a reader) is to start buying indie novels from B&N or other places.
I'm worried about Amazon exclusivity myself. But as long as the worried people just
stay worried and don't start
proactively buying from other places (i.e., buying on a protracted, regular basis), indie authors are going to continue posting low sales from B&N et. al. and other indies will sign up for Amazon exclusivity thinking "What's the downside?"
I've been trying to buy an indie novel a day from B&N, just to sort of vote with my wallet. I'm probably getting a lot of duds, but for ~$1-3 a day, to me, it's worth it. But I'm just one person and I only like certain genres.
If you don't want Amazon to become a monopoly:
Buy things elsewhere!
I hope that doesn't sound harsh, but for me that's just the way of things.