Please explain why the Beta 87A can not have a monitor placed directly in front of the user.
Because the Beta 87A has a supercardioid polar pattern. For the best explaination, download the following booklet:
Microphone Techniques for Music - Live Sound Reinforcement
Page 10 of the book has a section called "Using Polar Patterns to Reject Unwanted Sounds." This section has a great explanation and diagrams to answer your question.