Tainted is a terribly flawed concept unfortunately. At least the way it's implemented normally.

Strings are never universally tainted. They're tainted for a specific purpose. One language will treat backticks as a string quote, another as a subshell substitution. One will think $ is ok, another will interpolate the string. But in most cases I've seen, tainted flag is just used for "Ah, we quoted the ' in the string - you're safe now" :-(