There's a reason for this. These symbols don't work as search terms because they can actually be used to modify your searches. If you put a + before a search term, you are making sure all of your search results have that word. If you put a - before a search term you make sure that results with that word will not appear.
Also, both the +, -, and = signs are used when you use google to solve math problems. Google can serve as a calculator.
For a more in depth guide to using the + sign in a google search, try this helpful video.
How to Use Plus Sign in a Google Search -- powered by eHow.com