I think you are confusing beta testers for Quality Assurance personal.

Beta testers are there to help cover all the testing, where as the QA personal would be the ones that have guidelines on what and how to test.

As a beta tester, we do try to cover as many of the new features as we possibly can, but we are also not paid, and have time limits of our own to work within.  Our main purpose is to test the software on as many different setups and platforms as possible, not to catch every possible issue in the program.

 - Noel