3 reasons:
1. There are perhaps billions of plastic bags floating around the world's oceans. These may be mistaken for food (such as jellyfish) by large marine creatures (such as sea turtles and cetaceans). If ingested, they can block airways and digestive tracts, leading to death.
e.g. "Several years ago a minke whale was found on a Scottish beach with 800kg of plastic bags in its stomach".
(
http://www.panda.org/about_wwf/what_we_d…
2. Dangerous to humans: "about 25 children in the United States suffocate each year due to plastic bags, most under the age of one year". (
http://en.wikipedia.org/wiki/Plastic_bag…
3. I don't want archaeologists of the future to think of this time as the Plastic Bag Age: "The biggest problem with plastic bags is that they do not readily break down in the environment, with estimates for the time it takes them to decompose ranging from 20 to 1000 years".
(
http://www.abc.net.au/science/features/b…
Hope this helps. :)