There are reasons to be offended by this book, but it never hid the fact that it's a reprint. Seems a poor reason to complain.
I'm not sure if that's the only reason it is getting those reviews. I think it is an excuse to give it one star, but I think that this may be a response to some of the "official" changes that WotC has been making and seem to be implying are official with this book.
Another option may be that people may FEEL that these are official changes from a review I read where it said that Volo's and the old Mordekainen's were no longer allowed in official play and only this one would be allowed, thus enforcing people who play that way the need to buy this book, even if they own the previous books.
I'm not sure though.
If it is, it could spell trouble on the horizon.
If I recall, during the playtest WotC wanted to implement some items which were resoundly disliked by those playtesting, so they took them out.
Now, as time has passed, once again, they want to put those items back in that they wanted to originally. Instead of asking around or getting massive feedback as they did with the playtesters, they are simply saying they know better than the players (though the players may have constructive feedback if asked en masse that would have broader appeal).
This is relatively new though, and the reviews are still few in number. It could bounce back in it's rating.
If it does NOT though, it could be a reflection more on WotC's slant on things (the idea that there is no backwards compatibility with older books and thus one must buy the new one, feelings about other changes) regarding the ideas for the Anniversary edition, and it is NOT good.
I'd give it 3 months. If it bounces back, it's all good.
If it doesn't, and these things reflect what WotC is planning for the anniversary edition (the idea of planned obsolescence of all older material, changes they made without a massive playtest that were rejected once before, but they are now going ahead and including it anyways as the default), I think it might be wise to quickly go back to the drawing board and reassess what they are doing rather than push forward. This is just one release and easily gotten over, but if this is the true reaction to MotM, then the Anniversary edition is going to be HIGHLY controversial possibly, and could cause a decline in D&D sales and enthusiasm when it comes out. That SHOULD be troubling...BUT (that but is always there)...
Once again, this could just be a few naysayers who got onto Amazon reviews early on and are causing trouble. It could be that once more people have the book and review it for what it is, that review rating could go back to what is expected.