Why are Plastic Bags a problem ?

Because they are a waste problem that harms the environment:

Every year 4 million New Zealanders use 1 billion plastic shopping bags and 20 million Australians use approximately 5.6 billion.
A person's use of a plastic check-out bag can be counted in minutes - however long it takes to get from the shops to their homes. Plastic bags however, can take between 15 and 1000 years to break down in the environment.
In the marine environment plastic bag litter is lethal, killing at least 100,000 birds, whales, seals and turtles every year. After an animal is killed by plastic bags its body decomposes and the plastic is released back into the environment where it can kill again.
A Bryde's whale died on a Cairns beach after ingesting 6 square metres of plastic - including plastic bags. Such obstructions in animals can cause severe pain, distress and death.
On land, plastic bag litter can block drains and trap birds. They also kill livestock. One farmer near Mudgee NSW, carried out an autopsy on a dead calf and found 8 plastic bags in its stomach. The loss of this calf cost the farmer around $500.
Plastic bags are not free to consumers - they are actually adding an estimated NZ$25 million and A$173 million a year to New Zealand and Australia's grocery bills.
The amount of petroleum used to make one plastic bag would drive a car about 115 metres. The 6.9 billion plastic check-out bags we use every year is enough to drive a car 800 million kilometres or nearly 20,000 times around the world.
At least 16 million plastic bags end up as litter on our beaches, streets and parks. New Zealand local and State Governments spend $$ millions a year picking up litter.
Not all litter is deliberate. 47% of wind borne litter escaping from landfills is plastic - much of this is plastic bags.
Over 40,000 plastic check-out bags are dumped in landfills every hour in New Zealand and 200,000 are being dumped in Australia
Only 5% of Australia's plastic bags are currently being recycled, despite recycling facilities being available at major supermarkets.
In many council areas, plastic bags are the single main contaminant of kerbside recycling.
Plastic bags are a by-product of the oil industry. 5 billion HDPE plastic bags are imported into New Zealand and Australia every year.
Since March 2002, Ireland has reduced its plastic check-out bag usage by 90% and in April 2003 Coles Bay in Tasmania successfully banned plastic check-out bags in all their retail stores. In the first twelve months, Coles Bay stopped the use of 350,000 plastic check-out bags.
In Australia, over 10 million reusable bags have now been sold by Coles, Woolworths and Safeway stores. At only $1 each, they're a cheap way to save Australia's wildlife!

The original source of that info ie "the amount of petroleum used to create 1 plastic bag would drive a car 115 metres" was planet ark australia however they seem to have removed it from their site. It is however referenced on many other sites - here are some links:

http://www.reusablebags.com/news.php?action=details&id=193

http://www.litter.vic.gov.au/www/html/323-why-plastic-bags-are-a-problem.asp

http://fallscreek.com.au/environment/plasticbags.asp

http://www.ryde.nsw.gov.au/news/mr171003.htm