It's harder because in most places they're free, with no external incentive to avoid using them. Many of us bring bags to stores with us. Those of us with unconscious competence at it can't believe people use as much plastic as they do, but we still once used them.
Even the least environmentally aware people know that plastic bags pollute, kill wildlife, and take resources to create. We want to use less of them. We know financial incentive would help.
Should plastic bags be free?
How much should plastic bags cost?
The reflexive answer to how much plastic bags should cost is to add the cost of materials and labor in making them, but we know that answer is insufficient.
For one thing, markets set prices for many goods and services by supply and demand. So maybe the cost should vary with supply and demand.
But supply and demand isn't sufficient either, because stores can give things away to draw customers in. That's their right, isn't it?
Speaking of rights enters government territory. Even the most ardent advocates for smaller government, with rare exception, acknowledge the value of laws regulating how people affect each other. The standard illustration is that you are free to swing your fists however you like by yourself, but nearly everyone accepts regulation for swinging it into my nose.
Your plastic bags affect everyone
Plastic bags will last longer than you will--centuries. We universally agree, as far as I know, that they pollute, destroy wildlife, and consume resources to produce. You accepting a plastic bag means others will have to deal with your pollution emerging from using it.
In other words, using plastic bags means someone beside the user has to clean them up. Whatever you pay for them, someone else incurs a cost.
In the case of water and electricity, we pay for costs incurred before we receive them. We know laying pipes and wires cost money, so we accept paying them.
Is paying to clean pollution just paying future clean-up costs?
It seems fair to me to regulate that plastic bags should cost something non-zero.
Speaking of fairness suggests to me what makes things feel fair, which tends to mean whatever answer we go with result from a process people feel took their interests into account, which usually means it resulted from a reasonably democratic answer.
The answer to me, then, is that costing zero didn't result from a process that took everyone's interests into account. It resulted from people not having to account for costs they impose on others.
It emerged from a system based on a belief that humans couldn't damage the environment on a global scale. The evidence has shown that belief false.
It seems fair to me for most governing bodies to put to vote to regulate the cost of plastic bags. While I would accept any result arrived at democratically, it also seems fair to levy a non-zero tax, likely large enough for people to notice it.
The more I think about it, the less acceptable or fair it seems to allow plastic bags to cost nothing. While "acceptable" and "fair" are subjective, many places have already voted for and implemented those taxes.
I predict taxes on plastic bags be the global norm within a few years. Plastic bottles and so on as well.