I'm not a curl expert, but I think it's because the URL has parameters - if params are present, you have to treat the URL as a string. I'll follow up with the keeper of the docs to see if we can alter the example curl request to avoid future confusion.
Found the reason - it depends on the type of console you're using. Bash tries to turn ? and & into commands. So, on some terminals quotes aren't required, but they are in others.
I had the doc updated still, but, that's the reason.