Because there will probably be bugs that make it possible to break out of the sandbox and run arbitrary code on the target machine. Just like Java applets.
Do people actually read docs any longer? https://github.com/WebAssembly/ has some, my blog covered the 1VM requirement. There won't be a new "sandbox". JS and wasm interoperate over shared objects.
No, not really. People read headlines and make pithy middlebrow comments. Though I also don't think that's a new phenomenon...