In the case of banning plastic bags, certain areas have seen an uptick in feces in areas because the homeless were using the bags for defecation and sanitation. The ban he had an impact, detrimental to the point where a first world nation is on track to have preventable disease outbreaks. There are cost benefit analyses that should be done prior to saying that a ban is better than nothing.

Homeless sanitation is not independently sufficient justification to support heaps of littered disposable plastic bags everywhere.

There are many other possible ways to address this problem.

And yet none of those will be done, but we'll still continue with the ban. After all, it's the thought that counts.

Imagine that “free plastic bags” had never been a thing.

There are homeless people without adequate toilets. Would you suggest “let’s fill our world with heaps of littered plastic bags, because some homeless people might be able to use them as portable toilets” as a remedy?

No, that would be an absurd solution.

Yeah, instead we would have no solution at all and would just consider the higher amount of disease to be just a part of life. Homelessness isn't a new problem, it's simply one we don't do anything about. So these ideas of "we could do X to fix it" don't matter. We're not going to do them.