Disagree with this strongly. Public betas mean better quality final releases. You know that. We all know that Flash has had some quite buggy releases in the past, and that you rarely put out more than one bugfix release. Put off the profits for a few months and give your customers a chance to help you make a successful product.
I'll preface this with the fact that I'm not a Flash developer at all, but this is disturbing news to me especially since the announcement of a public beta was one of the "big" announcements at Adobe MAX 2009. However, I totally agree with the comment that was left on the blog post.
This is saddening news for a lot of people and the blog post cites the reason as "we want to make sure that we can provide the earliest possible delivery of the final software to the large number of designers and developers." I can only fathom "earliest possible delivery" means we need sales of this product now. Adobe is a for-profit company so they do need to make sales, however the decision to not have a public beta will probably hurt the quality of the product in the long term. This is an interesting dichotomy -- Flash CS5 will have no public beta whereas Adobe ColdFusion Builder went into Beta 3 on the same day. I hope the CF team continues with their public betas which is the right step in my opinion.
Personally, I believe the sooner you can deliver alpha/beta code to customers the better the product will be. This has always proved true for Mach-II (the open source project I'm involved in). The team can shake out a lot of bugs themselves, but it's always our users that find the interesting (and sometimes frustrating) edge cases. More brains are always better than fewer.
In all reality, I hope the Flash Platform Team at Adobe does well without the beta. Only the final release will tell if people will spend big money for the product. Remember, nobody likes paying for bugs!