Well, at this moment, there are basically three sources for previews and rules bit.
The Gleemax blog posts of the designers. They don't always contain useful information, but sometimes they do. It's free.
The online Dragon & Dungeon magazine on their website. That's free, and will stay so until D&D 4 really hits the shelves, and there is material each week.
If this material wasn't available online, it would have been in the paper magazine, which is not free.
The Preview Books. They're not free.
So, you have 3 ways to get information. The preview books are for those that have the money and are willing to spare it for the books. The rest is for everyone, and it's not too little. In some ways, it can be better, because the information is "fresh" and up to date.
The preview books have the nice advantage of being something you can hold in your hand and put in your shelves. If it was inside magazines, I'd might have dozens of magazines in its place, each containing only tidbits of the information I'd want to keep.
The preview books are not everyone. In some ways, you might consider them a "fan article". Some people buy T-Shirts from their favourite sports team or musician - that will also serve as a good "commercial" for the team or the band, and it is an important revenue stream for them. It's also a bit like buying the "Special Deluxe Version" of a movie that - aside from the movie - contains scenes that ended on the cutting floor, interviews and commentary. Nothing of that is necessary for the movie, but some people think it's worth paying extra money for (and in fact, some critics see them as an important part for rating the DVD for a movie - at least my favourite TV magazine does.)
What would be bad if WotC would _only_ sell the preview books and give no other information. But they aren't that stupid (yet

).
This wouldn't work anyway, because there just are never enough fans that are willing to buy it (compared to the "real deal"). But that doesn't mean they shouldn't put out something for these, too. A demand seems to exist, so why not serve it?
I think this also points out that the preview books are not a brochure or a commercial for D&D 4. It's extra material. It's the "Directors Commentary". You can't sell a commentary in hope of motivating people to see a movie or buy a DVD. But you can sell the commentary to people that are interested in the movie or DVD.