Why doesn't everyone know it's unsafe? I think because it's not something that's really pushed as an issue. In that NHS link, for example, it's mentioned once in amongst loads of other information and they only mention 'blankets' so people probably think a lighter cloth like a muslin is ok.
The Which link is much better, but I've only seen that because you've shown me, it wasn't within my algorithm even though I get almost exclusively baby content.
So, in conclusion, people don't know because they haven't been told. But, I agree that they should be told!