Just out of interest, why are you expending so much energy criticising a product you have no intention of buying?
I mean... I have no intention of getting my nose pierced... but I don’t go on an open assault on tattoo parlours?
When I first engaged, I had not yet decided to not buy. But my current decision to not buy doesn't mean that I don't have valid criticisms or that I should just shut up about it. Others can find value in my criticisms, which are, ultimately, pretty darned mild as these things go. I haven't maligned WotC, or the product at large, but kept to specific issues I see with how it's put together -- issues that have gone directly to my decision that I'll be skipping this one.
As for your tattoo example -- this is a poor example. The counterpart to this would be complaining about WotC even if you never, ever purchase or engage with any of their products. And, even then, depending on your criticism it can still be valid. Being a consumer, or a purchaser of a specific product or service, is not required to have good or valid criticism. I'm not on Twitter, should I not be able to complain about it at all, despite the fact that Twitter influences my life indirectly? Same here. I've investigated Frostmaiden -- I've read the excerpts printed online, I've read numerous synopses by those that have purchased the product, and I've participated in discussion. None of the premises I've based my opinions on have been refuted -- what I'm complaining about it actually in the book. That it, and other complaints I haven't presented in full because they are much more preference based, have led me to not purchase has absolutely no impact on the validity of my criticisms.
Further, you have mostly not been addressing my criticisms but instead attacking the validity of even being able to make them. You do so again, here. You're not actually trying to defend the product by showing how the criticism doesn't apply or can be mitigated but instead by trying to argue that I shouldn't be able to criticize the work at all, or, if I do, no one should listen because I'm not a purchaser. I continue to respond, in part, because I find that line of attack to be worth contesting.
But, to answer your question, I provide my criticism because others have before me. I've found good, reasoned criticism, even that I disagree with, to be some of the most enlightening aspects of examining my hobby. Had people been silent with criticism on a product they've chosen to not purchase, then my experience would have been lesser. So, providing good criticism is, to me, a valuable thing to do. Note that I've been very supportive of WotC, and understanding of how the issues I'm criticizing can happen, but I'm also of the opinion that if others never hear this criticism, they my purchase and be disappointed. Whereas, now, they may still purchase, but having been informed of possible potholes, now have a much better experience. This is my goal. What is your goal in preventing criticism?