Or... here the problem is that the "be liberal in what you accept" design principle failed to be captured by the HTTP specification writer, that forced a single SP character. It looks like a specification issue to me to use a syntax which is very prone to errors, and is even not much visible (you can't easily inspect double spaces in protocol traces when checking just with your eyes), and then be strict about it. Even changing separator, if you want to be strict, already helps, like in "foo|bar|zap" compared to "foo bar zap".
Humans are strage: many will spot "foo||bar|zap" as an error, but not "foo bar zap" as an error as serious as the previous one.
For it to hold up, you need to provide the further argument that you frequently need to switch from liberal to strict.