Easy answer...cause it is "trading" and not investing. Stock is sold to INVEST in a company. With shorts and puts, it skews the market, the consumer is no longer investing in the company.
Why keep them out of the market? Well Lehman, Goldman Bear Stearns and others put together mortgage default swaps. They bundled these sh!tty loans, got a better rating on them and knew they would default. They get paid on the front end for putting the funds together and being kind enough to sell them to the Norwegian teachers retirement fund claiming a high yield. Then they knew the assets in that fund would default, they shorted them. So who holds the bag? The consumer. Then the federal government had to bail them out.
It would like allowing me to be in a court room as an observer and bookie, taking bets on the verdict. Then eventually either the defense attn or prosecuting attn wants in on the bigger pay day, and justice goes out the window or perceived justice and it becomes a betting sporting event. You are right, I am shortsighted.
The majority of short sellers are hedge funds who are getting against a company similar to one they own as a way of protecting against the downside. Owning Home Depot and shorting Lowe's as an example. The majority of folks who own puts use them as insurance in case the stock falls. I don't see this as a bad thing.