This question already has an answer here:
I understand that best practice is to use tokens to prevent CSRF, but why do browsers permit cross-site POST requests in the first place? It seems like giving untrusted parties unfettered write access to your server is a bad idea.
The W3's site says that "Under the same-origin policy, cross-site sending of information is also dangerous since it enables attacks such as cross-site request forgery (CSRF) and clickjacking. The same-origin policy cannot address these security vulnerabilities in the same way it does those around receiving of information since prohibiting cross-site sending of information would prohibit cross-site hyperlinks." 
But this seems like a false dichotomy. We could disallow cross-site POSTs while still allowing hyperlinks, which are GETs.