While this gives the public a sneak peak at what they're working on, it can cause problems. Some betas are riddled with bugs. There are no manuals, tutorials, or anything that might help the user determine what the problem could be. If a beta trashes your System Folder, you are on your own to fix it. There are horror stories all over the Internet of people having problems with beta software.
Some people pay for download time.
By downloading an update off the Internet while paying for online time, you are paying for it. Many people feel this is wrong. Users should not have to invest their time and their money to fix a problem that is the software vendor's responsibility. The overnight popularity of downloading software updates and betas has created a new way of life for many computer users. We spend an inordinate amount of time trying to research, contribute, download and update.