You're surprised that GitHub allows people to host arbitrary repos?! Do you really prefer that GitHub would go "I'm sorry Dave, I'm afraid I can't let you do that"?
"You're under no obligation to choose a license. However, without a license, the default copyright laws apply, meaning that you retain all rights to your source code and no one may reproduce, distribute, or create derivative works from your work."
I have read the quoted GitHub docs page before and also found it somewhat odd. Not because it shouldn't be allowed to post public code without a LICENSE (or with a restrictive one), but because GitHub has a "Fork" button on every repository. It's strange to me that GitHub has a one-click button that can violate the default terms of code uploaded to the site.
Because that use case feels pretty far from the typical one for a public GitHub repo. Even when it was intended, having reliable metadata indicating that fact would be nice.
Absolutely agreed, but there's a vast difference between "would be nice" and "should require". I for one strongly prefer to avoid putting up any additional barriers to sharing, even at the cost of the default value being all rights reserved (which is a sensible default).
This is similar to saying that posting code anywhere online is useless. Not everyone is trying to start a collaborative project. Sometimes people just use github to showcase code, because it's a convenient platform.
Now you really are being silly. Most content on the internet doesn't have an explicit license. It's in no way legally dubious to look at it. Code is not special in this regard.