Making 10m requests to Google, asking for a CSAM image etc are all currently possible with an <img> tag. This does not prevent it in any way. Allowing GET requests would be a very lame proxy service, especially considering that the proxy falls when you close the tab, and that I mentioned the requests should only be allowed cookie-less (credentials: omit).

I think it's much more about open web and letting the user decide than about being cheap.