Conservatives often point out that laws, no matter how benign they may appear, have unintended consequences. They can reverberate in ways that not many people foresaw and nobody wanted: Raising the minimum wage can increase unemployment; prohibition can create black markets.

The efforts in many cities to discourage the use of plastic bags demonstrate that such unintended consequences can be, among other things, kind of gross.

San Francisco has been discouraging plastic bags since 2007, saying that it takes too much oil to make them and that used bags pollute waterways and kill marine animals. In 2012, it strengthened its law. Several West Coast cities, including Seattle and Los Angeles, have also adopted bans for environmental reasons. The government of Washington, D.C., imposes a 5 cent plastic-bag tax. (Advocates prefer to call it a “fee” because taxes are unpopular.) Environmental groups and celebrity activists, including Eva Longoria and Julia Louis- Dreyfus, support these laws.