Starting next week, Android developers are going to be required to submit new or updated apps with one of these ratings to have their app enter the market. If they are not updating their app developers have “several weeks” to submit a rating. If they do not submit a rating, their app will receive the “Mature” rating by default.
Here's what Google said on their Android developer blog:
Starting in a few weeks, we will be showing content ratings for all applications on Android Market. This new capability will provide users with additional information to help them select the best applications for them. [...]Speaking of upgrades to the Android Market, one thing that application developers would dearly love is an iTunes-like desktop application to browse and install apps.
We are working hard to rapidly deliver improvements and upgrades to Android Market. Please look for more Android Market upgrades in the coming weeks.
It's true, however, that Appbrain has come up with a great system and website to plug that gap. If you go to the Appbrain site, you can easily search and find great apps. Prior to the latest revision of the Android Market, you could even push applications onto your device, as well, by clicking the Install button on the website. Hopefully that functionality will be re-enabled by Google in the future.