Martha Stewart didn't go to jail for having any specialized knowledge, she went to jail because someone tipped her off that a company she was invested in was about to go south. She sold those shares to someone who had no reasonable way of knowing what she knew which is essentially a type of fraud.
And also because people think it's "unfair", like the stock market's supposed to be a game or something rather than a place that you can buy and sell stock for a price which is well-correlated with the company's prospects. :P
The ban on inside information is intended to attract capital by ensuring that stock markets are fair and transparent. If the investing public believes that the stock market is nothing more than a con to take rubes' money capital will stay away, and keep capital from flowing to productive firms.
No way - now 401ks are the only retirement savings vehicle aside from striking it rich through lottery or stocks. Guess how many people translate their investment in a mutual fund to the actual stock market shenanigans?