Yes, you're over-reacting. Your post sounds spiteful, and your threat of bad publicity sounds more like revenge than warning your fellow consumers. The way I see it, it would've been nice of them to give you a free MEAP copy, but it was well within their rights to refuse. Similarly, it's well within your rights to write a nasty blog post about how Manning is a bunch of meanies, but it'd be nice if you let it go, admitted that you made a mistake and got on with your life. It doesn't seem to me like there's anything to be gained by taking the low road here, even if you feel that it would be fair.
I have a story that starts similarly to yours, but ends very differently. I discovered "The Joy of Clojure" from a post here on HN, and after some consideration, I decided to pre-order. I was familiar with Amazon, but not with Manning, so I checked them out a little. I read their page about MEAPs and preview releases and really liked what I saw, so I pre-ordered the e-book copy from them directly, got my MEAP copy immediately and started reading. Since then, I've purchased/pre-ordered a half dozen other titles from them (frequently on steep discount thanks to their daily deals and newsletter sales) and they've become my go-to publisher for tech e-books when O'Reilly doesn't have what I'm looking for. I can't help but feel that my life would be ever-so-slightly poorer if I had had a bad experience with them right off the bat and decided to avoid them in the future.
Regardless of which path you decide to take, I hope that you enjoy reading the book. As someone who's fairly new to Clojure and functional programming, I've found it to be a great way to get deeper into the language as well as expand my horizons by exploring concepts like laziness and immutability that I don't see often enough as a Python programmer. Happy reading!