Yes. If opening a webpage downloaded a script that permanently altered the browser adding or removing functionality without explicit user intervention or consent it would be a security incident. There is even a class of scripts that warrants a special name because of this exactly behaviour: malware.

Considering the more general case of scripts being downloaded and executed in the browser (javascript, for instance) the more apt analogy would be one being downloaded and executed in a system with NoScript installed.

Just like NoScript is a tool that gives its users the power to decide on a case by case basis which scripts are executed by the browser, Debian is a tool that gives its users the power to decide on a case by case basis which closed source binaries are executed by their system.

Preventing this choice in this context is a security incident.

Just use no-script.