I feel like that's been explained in the thread.
This is an attempt to get more people to subscribe to D&D Beyond by leveraging FOMO. If you -- like me -- are already a Master tier subscriber, you (and your players, just like me, and my players) benefit from this.
But there are people on this board for whom a D&D Beyond subscription is out of reach -- we have unemployed posters who are saving up their pennies to get these new books at some point, others who have huge medical bills they're dealing with, etc. So they get to stand on the sidelines with the rest of the folks who can't or won't pay for D&D Beyond's most expensive tier and get to watch everyone else talk about the new game.
In addition, anyone with a streaming subscription -- or who's been within listening range of a radio station that has heard of Cory Doctrow -- knows that subscription fees start off reasonable, offering a lot of services, and then get more expensive and less robust over time. (Amazon Prime, already a pretty marginal streaming service, owned by one of the richest people in world history, now has ads, because why not?) So there's the concern that people are being herded into D&D Beyond so that they can get squeezed for increasing fees in the years to come.
These books aren't going to change in the two weeks between going on D&D Beyond and showing up in game stores -- there's just no time for that. So this isn't a "beta test" or anything of the sort. This is just a "pull out your wallet and pay an additional fee so that you can read the books early and participate in the conversation ahead of time" tax.
If this doesn't bother you, cool. There's no obligation to say "nuh uh" to each of these posts.
If you're someone who gets genuinely upset about people criticizing a corporation you have fond feelings for, use the ignore feature. The number of people making these complaints is relatively few in number. I've used ignore on people I view as completely unreasonable on both sides of the issue and it's made my ENWorld experience much better. I strongly endorse it, even if it means you put me on ignore as well.