> If the source is available to the public it is open source.
That's not the usual definition. The usual definition of "open source" revolves around having an open source license.
> I may not have a copyleft license, but for that we have other terms like "Free Software".
"Free Software" and "open source" are approximately equivalent (the FSF "free software" definition and the OSi standards for "open source" aren't identical, and there are probably some license that meet one but not the other, but they are close enough in practice that its not a huge difference.
"Copyleft" is a much narrower term than either "open source" or "free software". Most open source or free software licenses are not copyleft.
If you trust native code simply because it's open source, why don't you trust browser-interpreted code?