That's only true if everyone in the area is good about throwing away their trash in proper receptacles. That's the alternative stream that's difficult to control directly because it's at the individual level instead of the municipality and some people and areas don't give a damn about littering so the problem of plastic bags can be more pronounced depending on local culture about the environment.

The bag doesn't magically get from the store to the waste stream it has to be properly disposed of by the customer to make it there.