The grocers keep the cash from bag sales which is in reality is just a hidden tax increase. 
The grocers will show their gratitude by rebating some of it to the campaign coffers of the island's crooked politicians.
As usual, the consumers get the shaft.
The environment gets the shaft too because the same slobs who threw away their flimsy bags (which at least degraded quickly in the sun)  will now throw away the heavier bags which will last forever on the beaches and landscapes.
If anyone actually believes this issue about the environment, you are extremely naive and being played. 
That is because when corrupt politicians collude with cozy special interest group bank rollers, its always about the money.