What is the point? I'm confused.

Here in Australia, supermakets have always supplied plastic shopping bags to carry our groceries home in. I have always liked this idea, because they fit perfectly in my garbage bin as a liner, with handles. I know a lot of people who did this. These days with us all trying to be environmentally friendly, they are banning them and making us buy reusable shopping bags.
Now we have to try to remember to take the stupid reusable shopping bags to the shops in the first place, or we have to buy new ones when we get there. So, most people have thousands of them in the cupboard these days.
Then we have to spend more money in the supermarket to buy plastic garbage bags for our bins that are no different from the plastic shopping bags they used to supply us for free. How are we saving the planet by "buying" our plastic bags instead of getting them for free?
I will be very impressed and grateful if someone here can give me a logical explanation for this, as nobody on the planet has been able to do so, so far. Most people just look at me like I am an idot, laugh nervously and walk away.