In the web security space its typically called the browser security model and there are differences between browsers. The browser is not just a portal forwarding/displaying the intentions of the developers. If you want to see one of the more obvious examples look at the same origin policy and how it’s implemented across browsers.

Having said that, I do agree that browsers shouldn’t implement lots of crazy features but I personally don’t mind if they have some kind of malicious file scanning feature.

As an advanced user you can disable a lot of "helpful" features. Including safe browsing. Its in about:config

Browser.safebrowsing.malware.enabled

Just set it to false IF you consider yourself a poweruser.

How likely do you think it is that this has something to do with some of the organisations 'donating' to Mozilla? As anti-piracy lobbying for instance.