(And places like Germany only recently tightened laws against it.)
1) speeds up price discovery 2) makes complying with regulation cheaper 3) it is nearly impossible to enforce currently
Some op-ed pieces arguing for it.
http://www.marketwatch.com/story/why-insider-trading-should-...
http://www.forbes.com/sites/jeffreydorfman/2015/03/22/a-mode...
https://www.washingtonpost.com/news/wonk/wp/2013/07/26/insid...
I am on the fence myself, not having thought about the matter too much. Instinctively, I'd say insider trading should be handled as a breach of contract (ie if I trade on something that I signed an NDA for), not as a criminal matter.
(And places like Germany only recently tightened laws against it.)