The flimsy bags were all re-used as kitchen bin liners. The sturdy bags can't be used for that.
So I'm now having to buy single use plastic bin liners.
So what were they doing? Not lining their garbage bins? I find that unlikely. They were most likely buying plastic liners (ie. bags). In that case, the smart thing to do is to get them to switch to reusing their plastic bags, not making it harder for people who already reuse plastic bags to get bags.
>and their flimsy bags were ending up as litter
I'm not convinced that any meaningful amount of plastic bags end up as litter. They're used to transport something, typically to someone's home, where there's easy access to a garbage bin.