Tableau parameters and URL parameters are different. The latter is general web terminology. Filter Views using URL Parameters | Tableau Software
While there may not be a way to specify "All", you can certainly specify multiple values like .../WorldIndicators/GDPpercapita?Region=Asia,Africa,Middle%20East
I guess if the multi-valued filter defaults to All being selected, then you don't need to include it in the URL (or just leave the value blank .../WorldIndicators/GDPpercapita?Region= )